SI Units Explained: The International System of Units

Historical Development of the SI System

The International System of Units (SI, Système International d'Unités) is the modern form of the metric system and the world's most widely used measurement system. Its roots trace back to the French Revolution in the late 18th century, when France sought to replace its chaotic traditional measures with a uniform, decimal-based system. In 1795, France officially adopted the metric system, defining the metre as one ten-millionth of the distance from the equator to the North Pole along Earth's meridian. A platinum-iridium prototype meter and kilogram were made as standards in 1799, and the metric system's use spread to other countries over the 19th century.

To foster international agreement, the Metre Convention was signed in Paris in 1875 by 17 nations (including the U.S.), establishing the General Conference on Weights and Measures (CGPM) and the International Bureau of Weights and Measures (BIPM). This created a permanent, international structure to maintain and refine the measurement system. By the late 19th century, the centimeter–gram–second (CGS) system was in scientific use, but it proved inconvenient for some fields. In 1901, Giovanni Giorgi proposed adding an electrical unit to form an MKS (meter–kilogram–second–ampere) system. Following extensive international discussions, the CGPM adopted a set of base units: by 1954 the metre, kilogram, second, ampere, kelvin, and candela were all defined as base units. In 1960, the 11th CGPM unified these into the "International System of Units" (SI). Initially SI had six base units; a seventh base unit, the mole (for amount of substance), was added in 1971 to complete the system.

Over time, the SI base units have been redefined for greater precision and universality. Early definitions relied on physical artifacts or Earth properties (for example, the kilogram was long defined by a metal prototype cylinder). Advances in science allowed definitions to be tied to fundamental constants of nature. Notably, in 2018 the CGPM approved a major revision: effective May 2019, four of the base units (kilogram, ampere, kelvin, and mole) were redefined by fixing exact values of the Planck constant h, elementary charge e, Boltzmann constant k, and Avogadro's number N_A, respectively. The second, metre, and candela were already linked to constants (Cs-133 atomic frequency, speed of light c, and a standard optical frequency's luminous efficacy). Thus, all seven SI base units are now defined by fundamental constants, ensuring long-term stability and precision of the system.

The Seven SI Base Units and Their Definitions

All measured quantities in science and engineering are expressed in terms of seven SI base units. These base units and their physical definitions (as of the 2019 redefinitions) are:

• Second (s) – Unit of time. It is defined by an atomic reference: one second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine energy levels of the ground state of the cesium-133 atom. In essence, the cesium atomic clock provides the exact "ticks" of the SI second.
• Metre (m) – Unit of length. It is defined by the speed of light in vacuum. The definition fixes the speed of light at exactly 299,792,458 m/s, so one metre is the distance light travels in 1/299,792,458 of a second. This links the metre to the second and a universal constant of nature (c).
• Kilogram (kg) – Unit of mass. It is defined by fixing the Planck constant h = 6.62607015 × 10⁻³⁴ J·s exactly. Since 1 J = 1 kg·m²/s², this defines the kilogram in terms of the second and metre (via the joule, see below). The kg was once based on a metal prototype, but now is realized through precision experiments (such as the Kibble balance) that link mass to electrical and atomic standards via h.
• Ampere (A) – Unit of electric current. Defined by fixing the elementary charge e = 1.602176634 × 10⁻¹⁹ coulomb exactly. Since 1 C = 1 A·s, this means that one ampere is the current that corresponds to a flow of 1/(1.602176634×10⁻¹⁹) electrons per second. In other words, an ampere is the current that carries one coulomb of charge per second.
• Kelvin (K) – Unit of thermodynamic temperature. Defined by fixing the Boltzmann constant k = 1.380649 × 10⁻²³ J/K exactly. This means that a change of 1 kelvin corresponds to a change of 1.380649×10⁻²³ joules of thermal energy per degree of freedom. The kelvin is anchored so that 0 K is absolute zero. For everyday purposes, the degree Celsius (°C) is used for temperature: temperature in °C is offset from kelvin by t(°C) = T(K) − 273.15. Thus, 0 °C = 273.15 K, and a temperature difference of 1 °C is the same as 1 K in magnitude.
• Mole (mol) – Unit of amount of substance. One mole is defined to contain exactly 6.02214076 × 10²³ elementary entities (Avogadro's number). This could be atoms, molecules, or other specified particles. For example, 1 mol of carbon-12 atoms is 6.02214076×10²³ atoms, which by definition has a mass of exactly 12 grams (since the dalton, the atomic mass unit, is 1/12 the mass of a carbon-12 atom). The mole allows chemists to relate the microscopic scale (number of molecules) to macroscopic mass.
• Candela (cd) – Unit of luminous intensity (brightness of light in a particular direction). It is defined by fixing the luminous efficacy of monochromatic light at frequency 540×10¹² Hz (green light) as K_cd = 683 lm/W (lumens per watt). In practice, this means that a source emitting light of 540 THz frequency with a radiant intensity of 1/683 watt per steradian has an intensity of 1 candela. The candela ties optical power to the human visual response at that reference frequency.

These seven base units are dimensionally independent and form the foundation of the SI. Every other unit of measurement can be derived from them by multiplication or division. For example, the unit of area (square meter, m²) and the unit of speed (meter per second, m/s) are simple combinations of base units without special names. In total, the SI system is a coherent set: equations relating physical quantities hold true with no additional conversion factors when SI units are used.

SI Derived Units with Special Names

Beyond the base units, the SI includes many derived units for convenience, representing combinations of base units for specific physical quantities. By formulating combinations of the base units, we obtain derived units such as newton for force, pascal for pressure, etc. There are 22 derived units with special names and symbols officially recognized in SI. These derived units are coherent with the base units (no numerical conversion factors needed aside from unity). Below is a comprehensive list of the SI derived units with special names, their symbols, and definitions:

• Radian (rad) – unit of plane angle. It is a dimensionless unit defined as the ratio of arc length to radius, such that a full circle is 2π radians. (1 rad is essentially "1" as a pure number, with 2π rad = 360°).
• Steradian (sr) – unit of solid angle. Also dimensionless, it is the 3D analog of the radian. For example, a full sphere encompasses 4π sr.
• Hertz (Hz) – unit of frequency, defined as one cycle per second. 1 Hz = 1 s⁻¹. For example, a musical tone at 440 Hz means 440 oscillations per second.
• Newton (N) – unit of force. 1 N = 1 kg·m/s². It is the force required to accelerate a mass of 1 kg by 1 m/s². (Named after Isaac Newton's second law F = m·a.)
• Pascal (Pa) – unit of pressure or stress. 1 Pa = 1 N/m². It represents one newton of force spread over one square meter. For example, atmospheric pressure at sea level is about 101,325 Pa (101.3 kPa).
• Joule (J) – unit of energy, work, or heat. 1 J = 1 N·m = 1 kg·m²/s². One joule is the work done by a force of one newton moving an object one meter in the direction of the force. (Named after James Joule.)
• Watt (W) – unit of power (rate of doing work or emitting energy). 1 W = 1 J/s = 1 kg·m²/s³. A 60 W lightbulb consumes 60 joules of energy each second. (Named after James Watt.)
• Coulomb (C) – unit of electric charge. 1 C = 1 A·s. It is the amount of charge transported by a steady current of 1 ampere in 1 second (about 6.242×10¹⁸ elementary charges).
• Volt (V) – unit of electric potential (voltage). 1 V = 1 J/C = 1 kg·m²/s³·A⁻¹. It measures the potential difference that will impart 1 joule of energy per coulomb of charge. (Named after Alessandro Volta.)
• Farad (F) – unit of capacitance. 1 F = 1 C/V = 1 kg⁻¹·m⁻²·s⁴·A². A capacitor of 1 farad will increase in potential by 1 volt when charged with 1 coulomb. (Named after Michael Faraday.)
• Ohm (Ω) – unit of electric resistance. 1 Ω = 1 V/A = 1 kg·m²/s³·A⁻². A resistor of 1 ohm allows a current of 1 ampere to flow with a voltage of 1 volt across it. (Named after Georg Ohm.)
• Siemens (S) – unit of electric conductance. 1 S = 1/Ω = 1 A/V = 1 kg⁻¹·m⁻²·s³·A². It is the reciprocal of resistance; a 5 S conductance corresponds to 0.2 Ω resistance. (Named after Werner von Siemens.)
• Weber (Wb) – unit of magnetic flux. 1 Wb = 1 V·s = 1 kg·m²/s²·A⁻¹. For example, passing 1 weber of flux through a coil of 1-turn induces 1 volt when the flux is reduced to zero in 1 second. (Named after Wilhelm Weber.)
• Tesla (T) – unit of magnetic flux density (magnetic field strength). 1 T = 1 Wb/m² = 1 kg/s²·A⁻¹. One tesla is a strong magnetic field (approximately 20,000 times Earth's magnetic field). (Named after Nikola Tesla.)
• Henry (H) – unit of inductance. 1 H = 1 Wb/A = 1 kg·m²/s²·A⁻². A 1 H inductor produces an EMF of 1 V when the current through it changes at 1 A per second. (Named after Joseph Henry.)
• Degree Celsius (°C) – unit of Celsius temperature. The degree Celsius is technically not a separate SI unit (it uses the kelvin's magnitude), but it is a special name allowed for expressing temperature in a form familiar to daily life. The conversion is linear: T(°C) = T(K) – 273.15, so a temperature difference of 1 °C is the same as 1 K. For example, water freezes at 0 °C (273.15 K) and boils at 100 °C (373.15 K) under standard conditions.
• Lumen (lm) – unit of luminous flux. 1 lm = 1 cd·sr. It measures the total "amount" of visible light emitted by a source. A small LED flashlight might emit a few lumens, whereas a 60 W bulb emits around 800 lumens.
• Lux (lx) – unit of illuminance. 1 lx = 1 lm/m². It measures illumination (lumens per unit area). For instance, typical indoor lighting might be a few hundred lux, and direct sunlight about 100,000 lux.
• Becquerel (Bq) – unit of radioactivity (nuclear decay rate). 1 Bq = 1 s⁻¹. One becquerel corresponds to one decay event per second. (Named after Henri Becquerel.) This unit replaced the non-SI curie; 1 curie = 3.7×10¹⁰ Bq.
• Gray (Gy) – unit of absorbed dose of ionizing radiation. 1 Gy = 1 J/kg = 1 m²/s². It represents the absorption of one joule of radiation energy per kilogram of matter. (Named after L. H. Gray.)
• Sievert (Sv) – unit of dose equivalent (radiation effective dose). 1 Sv = 1 J/kg as well, also equating to J/kg, but applied to dose weighted by the biological effect of the type of radiation. (Named after Rolf Sievert.) For example, a dental X-ray might impart a few microsieverts.
• Katal (kat) – unit of catalytic activity. 1 kat = 1 mol/s. It is used in biochemistry to measure the activity of enzymes or catalysts (how many moles of reactant are converted per second). For instance, an enzyme with activity 1 kat converts one mole of substrate per second.

These derived units with special names make it easier to express measurements without using cumbersome base unit combinations. It's worth noting that many other derived units exist without special names (e.g. velocity in m/s, or concentration in mol/m³), but the above are the ones given unique names and symbols in SI for convenience. All of them are coherent with the base units, meaning their definitions are directly tied to the base unit definitions without additional conversion factors.

SI Prefixes for Decimal Multiples and Submultiples

To express very large or very small quantities, the SI uses a set of standard prefixes that represent powers of ten. Each prefix has a name and symbol, and prepending it to a unit creates a new unit scaled by that factor. For example, kilogram (kg) uses the prefix kilo- (k) to indicate 10³ grams, and millimeter (mm) uses milli- (m) to indicate 10⁻³ meters. Using prefixes avoids having to write out inconvenient numbers; one can say "500 nanometers" instead of "0.0000005 meters". The SI prefixes range from extremely large (10³⁰) to extremely small (10⁻³⁰).

Factor (10n)	Prefix	Symbol	Example
1030	quetta	Q	1 QB = 1030 bytes
1027	ronna	R	1 RJ = 1027 joules
1024	yotta	Y	1 YJ = 1024 joules
1021	zetta	Z	1 ZW = 1021 watts
1018	exa	E	1 EB = 1018 bytes
1015	peta	P	1 PB = 1015 bytes
1012	tera	T	1 TB = 1012 bytes
109	giga	G	1 GHz = 109 hertz
106	mega	M	1 MW = 106 watts
103	kilo	k	1 km = 103 meters
102	hecto	h	1 hPa = 102 pascals
101	deca	da	1 dam = 101 meters
10−1	deci	d	1 dm = 10−1 meters
10−2	centi	c	1 cm = 10−2 meters
10−3	milli	m	1 mm = 10−3 meters
10−6	micro	μ	1 μm = 10−6 meters
10−9	nano	n	1 nm = 10−9 meters
10−12	pico	p	1 ps = 10−12 seconds
10−15	femto	f	1 fs = 10−15 seconds
10−18	atto	a	1 am = 10−18 meters
10−21	zepto	z	1 zg = 10−21 grams
10−24	yocto	y	1 ys = 10−24 seconds
10−27	ronto	r	1 rm = 10−27 meters
10−30	quecto	q	1 qg = 10−30 grams

Each step represents a factor of 1000 (10³) except for the smaller deci (10⁻¹) and centi (10⁻²) and larger deca (10¹) and hecto (10²), which were historically adopted for convenience in certain cases (e.g. 1 deciliter = 100 mL). The largest prefixes, ronna- and quetta-, and the smallest, ronto- and quecto-, were added in 2022 by the 27th CGPM to address the growing needs for expressing data sizes and microscopic scales beyond 10²⁴. For instance, 1 quettameter (Qm) is 10³⁰ meters, and 1 quectogram (qg) is 10⁻³⁰ grams.

When using prefixes, one combines the prefix with the unit name or symbol as a single word or symbol, without space (e.g. "kilometer" or "km"). A few notes: prefixes are case-sensitive (e.g. m = milli = 10⁻³, but M = mega = 10⁶). Also, because the kilogram already contains the prefix "kilo" in its name, prefixes are applied to the gram instead for mass (e.g. 1 mg = 10⁻³ g = 10⁻⁶ kg, not "microkilogram"). This table of prefixes allows extremely large or small measurements to be expressed succinctly, simplifying scientific notation.

Non-SI Units Accepted for Use with SI

The SI is a complete system, but some non-SI units are accepted for use alongside SI due to practical importance or widespread usage. These include certain traditional units for time, angle, volume, mass, etc., that can be used with SI values (they have exact definitions in terms of SI units). A few notable examples of accepted non-SI units and their relations to SI are:

Units Accepted for Use with SI Due to Practical Importance

• Time units – The SI base unit of time is the second, but larger time units are commonly used. The minute (min), hour (h), and day (d) are accepted for use. By definition, 1 min = 60 s; 1 h = 60 min = 3,600 s; 1 d = 24 h = 86,400 s. These units are used universally in daily life and are compatible with SI (for example, speeds can be given in km/h, since hour is recognized)
• Angular units – The SI unit for plane angle is the dimensionless radian, but traditional sexagesimal angle units are accepted. The degree (°), minute (′), and second (″) of arc are such units: 1° = π/180 rad, 1′ = 1/60°, and 1″ = 1/60′ (thus 1″ = π/648,000 rad)npl.co.uk. For instance, latitude/longitude and geometry often use degrees – one degree is about 0.01745 radians.
• Volume unit - The litre (L or l) is a non-SI unit of volume accepted for use with SI. 1 L = 1 cubic decimeter = 0.001 m^3. In other words, a liter is the volume of a cube 10 cm on a side. (Both “L” and “l” are allowed symbols; a capital L is often used to avoid confusion between the lowercase l and the numeral 1.)
• Area unit - The hectare (ha) is used for land area. 1 ha = 1 hectometer^2 = 10^4 m^2 (a square 100 m on a side). This unit is commonly used in agriculture, forestry, and land planning.
• Mass unit - The tonne (t), also spelled “metric ton,” is accepted as a unit of mass for large quantities. 1 t = 1000 kg. For example, freight, vehicle weights, and production quantities are often given in tonnes. (Note: 1 tonne ≈ 2204.6 pounds in the US customary system.)
• Energy unit - The electronvolt (eV) is a convenient unit of energy in atomic and particle physics. 1 eV = 1.602 176 634 × 10^−19 J. It represents the kinetic energy gained by an electron accelerating through a potential difference of one volt. Though small, it’s useful for atomic-scale energies (e.g. photon energies, ionization energies).
• Mass for atoms - The dalton (Da), also known as the unified atomic mass unit (u), is used in chemistry and biochemistry for atomic and molecular masses. 1 Da = 1.660 539 066 60 × 10^−27 kg, which is 1/12 the mass of a carbon-12 atom (approximately 1.67×10^−27 kg). This unit is accepted for use with SI to conveniently express atomic-scale masses (e.g. a protein might have a mass of 50 kDa instead of 8.3×10^−20 kg).
• Logarithmic ratio units - Units like the neper (Np) and bel (B) (and its decibel, dB) are used for logarithmic ratios (for sound level, signal gain, etc.). These are dimensionless with special names: for example, a power ratio of 10:1 is 1 bel (i.e. 10 dB). The bel and decibel are commonly used for sound intensity and electronics, even though they are not SI units; they are accepted for use with SI (the neper is another logarithmic ratio unit, using natural log base). [These units are a bit specialized; in daily practice decibels are encountered for sound, e.g. 60 dB SPL for normal conversation.]

Units Used in Specific Fields

• Energy - electronvolt (eV) – Used in atomic and particle physics, 1 eV ≈ 1.602176634 × 10⁻¹⁹ J.
• Mass - dalton (Da) or unified atomic mass unit (u) – Used in chemistry and biochemistry for atomic and molecular masses, 1 Da ≈ 1.660539040 × 10⁻²⁷ kg.
• Length - astronomical unit (au) – Used in astronomy, 1 au ≈ 149,597,870,700 m (the average Earth-Sun distance).
• Pressure - bar (bar) – Used in meteorology and fluid mechanics, 1 bar = 10⁵ Pa (close to standard atmospheric pressure of 101,325 Pa).
• Area - barn (b) – Used in nuclear physics for cross sections, 1 b = 10⁻²⁸ m².

Units Maintained for Practical Reasons

• Blood pressure - millimeter of mercury (mmHg) – Still widely used in medicine, 1 mmHg ≈ 133.322 Pa.
• Navigation - nautical mile (M or NM) – Used in maritime and air navigation, 1 M = 1852 m exactly.
• Speed - knot (kn) – Used in maritime and air navigation, 1 kn = 1 nautical mile per hour ≈ 0.514444 m/s.

While these non-SI units are accepted for use with the SI, they are not part of the SI proper. In scientific contexts, especially in publications, it is often recommended to include the equivalent SI value in parentheses when using these units. The acceptance of these units acknowledges both practical reality and historical tradition while maintaining the coherence and universality of the SI as the primary system of units.

Global Adoption and Usage of SI Units

Global adoption of the SI (metric system) is nearly universal. Over the 19th and 20th centuries, most countries switched from traditional measurement systems (imperial, customary, etc.) to the metric system, and eventually to SI as it became established in 1960. As of today, SI units are the official standard in almost all nations, enabling consistency in science, trade, and daily life. By 2011, it was estimated that 95% of the world’s population lives in countries where the metric system is the sole legal system of weights and measures.

• United States: The US uses a dual system. By law, the metric system is the “preferred” system for trade and commerce (since the Metric Conversion Act of 1975), and SI units are used in science, medicine, and many industries. However, customary units (feet, pounds, gallons, etc.) remain common in everyday use and certain sectors. The US government and military have largely metricated internally (for example, food nutrition labels use grams, and many manufacturing industries use millimeters). Notably, all US customary units are defined in terms of SI (e.g. 1 inch is defined as 2.54 cm exactly). In short, the US recognizes SI but has not fully transitioned in public life.
• Myanmar (Burma): Myanmar historically used traditional Burmese units, but in 2013 the government announced plans to adopt the metric system, and metrication has been gradually progressingen.wikipedia.org. For instance, Myanmar has begun using kilometers on road signs and liters for fuel. As of the mid-2020s, Myanmar is in transition to SI, though some local units persist in markets.
• Liberia: Liberia, which formerly used US-like units, has also expressed commitment to metrication. In 2018, Liberia’s government pledged to adopt SI officiallyen.wikipedia.org. Implementation is ongoing, with increasing use of metric units in trade and official matters.

It’s worth noting that even in non-metric countries, SI is present. For example, American scientists and engineers work in SI for virtually all technical purposes (NASA, for instance, designs spacecraft using metric units). Products in the US often have dual labeling (e.g. a soda bottle is 2 liters, and a car’s engine might be described as 5.0 liters). In Liberia and Myanmar, metric units are taught in schools and used in many imports. So, no country today is truly “SI-free.”

Some countries use a mixed approach. For instance, the United Kingdom and Canada officially adopted SI in the late 20th century, and most industries and official measurements there use metric units. However, the UK still commonly uses certain imperial units by tradition or law: miles and yards for road distances, pints for beer, and pounds and ounces alongside kilograms for body weight or produce. This makes the UK effectively bilingual in units. Canada uses SI for almost everything (road signs in km, weather in °C, etc.), but many people still reference heights or weights in imperial out of familiarity. Such partial usage often exists in countries that transitioned in living memory, but the trend over time is increasing use of SI.

Globally, the SI system’s uniformity is a tremendous advantage. It underpins international trade (a German car part described in millimeters can fit a Japanese machine without conversion issues), and it allows scientific results to be shared and understood worldwide without unit confusion. Virtually all international standards (in aviation, shipping, medicine, etc.) use SI units or SI-compatible units. Even in the United States, industries that export products must label in SI for the global market. Overall, the SI has achieved near-complete international adoption, making it a true global language of measurement.

Advantages of the SI System

The International System of Units (SI) offers numerous advantages that have led to its widespread adoption in science, engineering, commerce, and everyday life around the world:

Scientific and Technical Advantages

• Coherence: The SI is a coherent system, meaning that derived units are products and quotients of base units with no numerical factors other than 1. For example, 1 newton equals exactly 1 kg·m/s², with no conversion factor needed. This coherence greatly simplifies calculations and reduces errors.
• Universal constants-based definitions: Modern SI definitions are based on invariant constants of nature rather than physical artifacts or phenomena that might change over time. This ensures long-term stability and universality of the units.
• Precision and reproducibility: The quantum-based definitions allow for extremely precise measurements that can be reproduced anywhere in the world. For instance, the second can now be measured with an accuracy of about 1 part in 10¹⁶, equivalent to losing less than 1 second in 300 million years.
• Scalability: With its systematic set of prefixes ranging from quecto- (10⁻³⁰) to quetta- (10³⁰), the SI can express quantities across 60 orders of magnitude using the same base units, accommodating everything from subatomic particles to astronomical distances.

Practical Advantages

• Decimal-based system: The SI uses powers of 10 for all unit relationships, making conversions between multiples and submultiples straightforward. Converting between kilometers and meters (multiply by 1000) is much simpler than converting between miles and feet (multiply by 5280).
• Consistency in unit relationships: The relationship between units of the same quantity always follows the same pattern. For example, volume units (cubic meter, cubic centimeter) are always the cube of the corresponding length units.
• Simplified calculations: The coherent nature of SI units means that equations can be written directly with the quantities, without needing conversion factors. This reduces calculation errors and cognitive load.
• Comprehensive coverage: The SI covers all physical quantities needed in science, engineering, and everyday life, from fundamental quantities like length and time to complex derived quantities like magnetic flux density or catalytic activity.

Global and Economic Advantages

• International standardization: As the globally accepted system of measurement, the SI facilitates international scientific collaboration, trade, and communication. Technical specifications, research findings, and product information can be understood worldwide without ambiguity.
• Reduction of trade barriers: Common measurements reduce non-tariff barriers to trade and simplify international commerce. Products manufactured to SI specifications can be sold globally without modification.
• Error reduction: The consistency and simplicity of the SI reduce the likelihood of conversion errors that can have serious consequences. For example, the Mars Climate Orbiter was lost in 1999 due to a confusion between metric and imperial units.
• Educational efficiency: The logical structure of the SI makes it easier to teach and learn compared to systems with arbitrary conversion factors. Students need to memorize fewer conversion factors and can derive many relationships from first principles.

These advantages explain why the SI has been adopted by nearly all countries worldwide and is the mandatory system in scientific contexts. Even in countries like the United States, where customary units remain common in everyday life, the SI is the standard in scientific, medical, and increasingly, industrial applications. The system's foundation on universal constants ensures that it will remain relevant and precise far into the future, regardless of technological changes.

SI Units in Everyday Life and Practical Applications

While the SI was developed for scientific and technical purposes, it has become deeply integrated into everyday life in most countries around the world. Here's how SI units appear in our daily activities and various professional fields:

Everyday Applications of SI Units

• Shopping and Cooking:
  • Food is sold by mass in kilograms (kg) or grams (g)
  • Liquids like milk, juice, and cooking oils are measured in liters (L) or milliliters (mL)
  • Recipe ingredients are specified in grams, milliliters, and cooking temperatures in degrees Celsius (°C)
• Personal Health:
  • Body weight is measured in kilograms
  • Height is measured in meters or centimeters
  • Body temperature is measured in degrees Celsius (normal being around 37°C)
  • Medicine dosages are prescribed in milligrams (mg) or micrograms (μg)
• Transportation:
  • Road distances are marked in kilometers (km) or meters (m)
  • Speed limits are set in kilometers per hour (km/h)
  • Fuel efficiency is measured in liters per 100 kilometers (L/100 km)
  • Tire pressure is measured in kilopascals (kPa) or bars
• Weather and Climate:
  • Temperature is reported in degrees Celsius
  • Rainfall is measured in millimeters (mm)
  • Wind speed is given in meters per second (m/s) or kilometers per hour (km/h)
  • Atmospheric pressure is measured in hectopascals (hPa) or millibars (mbar)
• Home and Energy:
  • Electricity consumption is measured in kilowatt-hours (kWh)
  • Appliance power ratings are given in watts (W) or kilowatts (kW)
  • Light bulb brightness is measured in lumens (lm)
  • Room dimensions are measured in meters or centimeters

Professional and Industrial Applications

• Medicine and Healthcare:
  • Patient vital signs: temperature (°C), blood pressure (mmHg, though not SI), weight (kg)
  • Pharmaceutical dosages in milligrams or micrograms
  • Medical imaging radiation doses in grays (Gy) or sieverts (Sv)
  • Blood chemistry measurements in moles per liter (mol/L)
• Engineering and Construction:
  • Structural calculations using newtons (N) for forces
  • Material strength in pascals (Pa) or megapascals (MPa)
  • Building dimensions in meters
  • Electrical specifications in volts (V), amperes (A), and ohms (Ω)
• Manufacturing:
  • Component tolerances in micrometers (μm)
  • Material properties like density in kilograms per cubic meter (kg/m³)
  • Process temperatures in kelvins (K) or degrees Celsius (°C)
  • Production rates in units per second (s⁻¹)
• Environmental Science:
  • Pollutant concentrations in parts per million (ppm) or milligrams per cubic meter (mg/m³)
  • Water quality parameters in moles per liter (mol/L)
  • Soil properties in kilograms per hectare (kg/ha)
  • Radiation measurements in becquerels (Bq)
• Information Technology:
  • Data storage in bytes with SI prefixes (kilobyte, megabyte, gigabyte, terabyte)
  • Processor speeds in gigahertz (GHz)
  • Network speeds in megabits per second (Mbps) or gigabits per second (Gbps)
  • Display resolutions in millimeters (mm) for pixel pitch

Regional Variations

While the SI is the official system of measurement in most countries, there are regional variations in its everyday application:

• In the United States, everyday measurements often use customary units (miles, pounds, Fahrenheit), but scientific, medical, and increasingly industrial contexts use SI units
• The United Kingdom uses a mix of metric and imperial units in daily life (distances in miles, beer in pints, but food sold in kilograms)
• Canada and Australia have largely converted to SI but retain some imperial units in certain contexts
• Most other countries use SI units almost exclusively in both official and everyday contexts

The integration of SI units into everyday life continues to expand globally, driven by international trade, scientific education, and the inherent advantages of a decimal-based, coherent measurement system. Even in countries that traditionally use other systems, younger generations are increasingly familiar with SI units through education, international media, and global connectivity.

Conclusion: The Enduring Legacy and Future of SI

The International System of Units (SI) stands as one of humanity's greatest achievements in standardization and international cooperation. From its revolutionary beginnings during the French Revolution to its quantum-based definitions today, the SI has evolved into a sophisticated measurement framework that underpins virtually all aspects of modern civilization.

The 2019 redefinition of the SI base units marked a pivotal moment in metrology history, completing the transition from physical artifacts and Earth-based references to universal constants of nature. This quantum revolution in measurement science ensures that the SI will remain stable, accessible, and precise for generations to come, regardless of technological changes or physical conditions. The system now rests on the bedrock of invariant properties of the universe itself.

The coherent, decimal-based structure of the SI continues to demonstrate its superiority over alternative measurement systems through its mathematical elegance, ease of use, and adaptability across all scales of physical reality. From the smallest subatomic particles to the vast expanses of the cosmos, from everyday kitchen measurements to the most precise scientific experiments, the SI provides a common language that transcends national boundaries and scientific disciplines.

As global challenges like climate change, public health crises, and technological innovation demand ever more precise measurements and international collaboration, the importance of a unified measurement system only grows. The SI will continue to evolve as measurement science advances, but its fundamental principles of universality, coherence, and accessibility will remain constant.

The story of the SI is ultimately a human story—one of our species' drive to understand, quantify, and communicate about the physical world with ever-increasing precision. It represents a remarkable consensus among nations and scientific communities, demonstrating what can be achieved when we work together toward common standards that benefit all of humanity.

References and Further Reading

• Bureau International des Poids et Mesures (BIPM). (2019). The International System of Units (SI), 9th edition. https://www.bipm.org/en/publications/si-brochure/
• National Institute of Standards and Technology (NIST). (2019). SI Units - Base and Derived Units. https://www.nist.gov/pml/weights-and-measures/metric-si/si-units
• International Organization for Standardization (ISO). (2009). ISO 80000 Quantities and Units.
• Quinn, T. J. (2019). From Artefacts to Atoms: The BIPM and the Search for Ultimate Measurement Standards. Oxford University Press.
• Knotts, S., Mohr, P. J., & Phillips, W. D. (2017). An Introduction to the New SI. The Physics Teacher, 55(1), 16-21.
• Stock, M., et al. (2019). The revision of the SI—the result of three decades of progress in metrology. Metrologia, 56(2), 022001.