Science & Chemistry
SI Units Chart
Compare the seven SI base units, 22 coherent derived units with special names, common compound units, defining constants, dimensions, prefixes, notation rules, and non-SI relationships used across science and engineering.
Unit symbols are case-sensitive. For example, m means meter when used as a unit and milli when used as a prefix, while M means mega. Preserve capitalization, prefixes, powers, and temperature scales when copying a measurement.
How the International System of Units fits together
The BIPM SI Brochure is the authoritative description of the International System of Units. Its current 9th edition, version 4.01, was revised in June 2026.
The SI has seven base units: meter, kilogram, second, ampere, kelvin, mole, and candela. Derived units are formed algebraically from those base units, and 22 coherent derived units have special names and symbols.
A coherent derived unit introduces no extra numerical factor. One newton is exactly kg·m·s⁻², one pascal is exactly N/m², one joule is exactly N·m, and one watt is exactly J/s.
The modern SI is defined by seven fixed numerical values for defining constants, not by seven physical artifacts. NIST summarizes the same structure as seven base units plus 22 named derived units, giving 29 SI units with special names or base-unit status.
Base units
7
m, kg, s, A, K, mol, and cd are the seven SI base-unit symbols.
Named derived units
22
Special names include newton, pascal, joule, watt, volt, gray, sievert, and katal.
Defining constants
7 exact values
The modern SI fixes numerical values for seven defining constants.
Prefixes
24
Current SI prefixes span powers from 10³⁰ down to 10⁻³⁰.
Seven SI Base Units Chart
The seven base quantities and their SI base units. All coherent SI derived units can be expressed as products of powers of these base units.
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| Base quantity | Unit name | Symbol | Dimension symbol | Typical measurement |
|---|---|---|---|---|
| Length | meter | m | L | Distance, wavelength, dimensions |
| Mass | kilogram | kg | M | Mass of an object or sample |
| Time | second | s | T | Duration and time interval |
| Electric current | ampere | A | I | Electric current |
| Thermodynamic temperature | kelvin | K | Θ | Thermodynamic temperature |
| Amount of substance | mole | mol | N | Number of specified elementary entities |
| Luminous intensity | candela | cd | J | Luminous intensity in a specified direction |
SI base-unit symbols are m, kg, s, A, K, mol, and cd.
- • The base quantities are a convention for expressing the SI; modern unit definitions are tied to fixed values of seven defining constants.
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Interactive reference
SI Unit Finder
Select one of the 29 SI units with special names—the seven base units plus 22 coherent derived units—to see its quantity, symbol, category, and relation to base units.
Selected SI unit
newton (N)
Quantity
force
Class
Derived unit with special name
Base-unit expression
kg·m·s⁻²
One newton is the force that gives 1 kg an acceleration of 1 m/s².
Seven SI Defining Constants
Since the 2019 revision, the SI is defined by fixing exact numerical values for seven defining constants.
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| Defining constant | Symbol | Fixed numerical value | Unit used in definition | Strongly associated unit |
|---|---|---|---|---|
| Cesium-133 hyperfine transition frequency | ΔνCs | 9,192,631,770 | Hz | second |
| Speed of light in vacuum | c | 299,792,458 | m/s | meter |
| Planck constant | h | 6.62607015 × 10⁻³⁴ | J·s | kilogram |
| Elementary charge | e | 1.602176634 × 10⁻¹⁹ | C | ampere |
| Boltzmann constant | k | 1.380649 × 10⁻²³ | J/K | kelvin |
| Avogadro constant | NA | 6.02214076 × 10²³ | mol⁻¹ | mole |
| Luminous efficacy of specified 540 THz radiation | Kcd | 683 | lm/W | candela |
The numerical values shown are exact by SI definition.
- • The SI is a connected system: each base-unit realization can involve several defining constants, not only the one listed as strongly associated here.
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Base units and defining constants are related, but not one-to-one
It is useful to associate the second with ΔνCs, meter with c, kilogram with h, ampere with e, kelvin with k, mole with NA, and candela with Kcd. But the SI is a connected system: practical realizations of one unit can depend on several defining constants and already realized units.
22 SI Derived Units with Special Names
These coherent derived units have special names and symbols. Their base-unit expressions reveal the dimensional relationship behind each name.
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| Quantity | Unit | Symbol | Equivalent SI expression | Base-unit expression |
|---|---|---|---|---|
| Plane angle | radian | rad | 1 | m/m |
| Solid angle | steradian | sr | 1 | m²/m² |
| Frequency | hertz | Hz | — | s⁻¹ |
| Force | newton | N | — | kg·m·s⁻² |
| Pressure, stress | pascal | Pa | N/m² | kg·m⁻¹·s⁻² |
| Energy, work, heat | joule | J | N·m | kg·m²·s⁻² |
| Power, radiant flux | watt | W | J/s | kg·m²·s⁻³ |
| Electric charge | coulomb | C | A·s | A·s |
| Electric potential difference | volt | V | W/A | kg·m²·s⁻³·A⁻¹ |
| Capacitance | farad | F | C/V | kg⁻¹·m⁻²·s⁴·A² |
| Electric resistance | ohm | Ω | V/A | kg·m²·s⁻³·A⁻² |
| Electric conductance | siemens | S | A/V | kg⁻¹·m⁻²·s³·A² |
| Magnetic flux | weber | Wb | V·s | kg·m²·s⁻²·A⁻¹ |
| Magnetic flux density | tesla | T | Wb/m² | kg·s⁻²·A⁻¹ |
| Inductance | henry | H | Wb/A | kg·m²·s⁻²·A⁻² |
| Celsius temperature | degree Celsius | °C | K interval size | K |
| Luminous flux | lumen | lm | cd·sr | cd·sr |
| Illuminance | lux | lx | lm/m² | cd·sr·m⁻² |
| Radionuclide activity | becquerel | Bq | — | s⁻¹ |
| Absorbed dose, kerma | gray | Gy | J/kg | m²·s⁻² |
| Dose equivalent | sievert | Sv | J/kg | m²·s⁻² |
| Catalytic activity | katal | kat | mol/s | mol·s⁻¹ |
Coherent derived units contain no additional numerical conversion factor.
- • Different quantities can share the same base-unit expression: Hz and Bq are both s⁻¹, while Gy and Sv are both J/kg, but the special names identify different physical quantities.
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Common Coherent Derived Units without Special Names
Many SI derived units are written directly as combinations of base units because they do not need a special unit name.
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| Quantity | SI unit | Symbol/expression | Dimension pattern | Example meaning |
|---|---|---|---|---|
| Area | square meter | m² | L² | Surface area |
| Volume | cubic meter | m³ | L³ | Three-dimensional volume |
| Speed, velocity | meter per second | m/s | L·T⁻¹ | Distance rate |
| Acceleration | meter per second squared | m/s² | L·T⁻² | Rate of change of velocity |
| Wavenumber | reciprocal meter | m⁻¹ | L⁻¹ | Spatial frequency |
| Density | kilogram per cubic meter | kg/m³ | M·L⁻³ | Mass per volume |
| Specific volume | cubic meter per kilogram | m³/kg | L³·M⁻¹ | Volume per mass |
| Current density | ampere per square meter | A/m² | I·L⁻² | Current per area |
| Magnetic field strength | ampere per meter | A/m | I·L⁻¹ | Magnetizing field quantity |
| Amount concentration | mole per cubic meter | mol/m³ | N·L⁻³ | Amount of substance per volume |
| Luminance | candela per square meter | cd/m² | J·L⁻² | Luminous intensity per projected area |
| Molar mass | kilogram per mole | kg/mol | M·N⁻¹ | Mass per amount of substance |
Coherent SI derived units are algebraic products of powers of base units.
- • A familiar practical unit can be non-coherent even when it is accepted or widely used; for example, liter is exactly 10⁻³ m³.
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SI Mechanics Unit Relationships
Mechanics units connect length, mass, and time through familiar equations for motion, force, pressure, energy, and power.
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| Quantity | Relationship | SI unit | Equivalent expression | Interpretation |
|---|---|---|---|---|
| Speed | v = distance/time | m/s | m·s⁻¹ | Length traveled per time |
| Acceleration | a = Δv/Δt | m/s² | m·s⁻² | Velocity change per time |
| Force | F = ma | N | kg·m·s⁻² | Mass times acceleration |
| Pressure | p = F/A | Pa | N/m² | Force per area |
| Work | W = F·d for aligned constant force | J | N·m | Energy transferred by force through distance |
| Power | P = E/t | W | J/s | Energy transfer rate |
| Momentum | p = mv | kg·m/s | kg·m·s⁻¹ | Mass times velocity |
| Impulse | J = FΔt | N·s | kg·m/s | Change in momentum |
| Density | ρ = m/V | kg/m³ | kg·m⁻³ | Mass per volume |
| Dynamic viscosity | τ/(du/dy) | Pa·s | kg·m⁻¹·s⁻¹ | Resistance to shear flow |
Symbols used in equations represent quantities; unit symbols identify measurement units.
- • The same letter can represent a quantity in one context and a unit symbol in another, so typography and context matter.
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SI Electrical and Magnetic Units Chart
Electrical and magnetic quantities use several derived units with special names. Their equivalent relationships make the system easier to reconstruct.
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| Quantity | Unit | Symbol | Useful relation | Base-unit expression |
|---|---|---|---|---|
| Electric current | ampere | A | Base unit | A |
| Electric charge | coulomb | C | A·s | A·s |
| Potential difference | volt | V | W/A | kg·m²·s⁻³·A⁻¹ |
| Resistance | ohm | Ω | V/A | kg·m²·s⁻³·A⁻² |
| Conductance | siemens | S | A/V | kg⁻¹·m⁻²·s³·A² |
| Capacitance | farad | F | C/V | kg⁻¹·m⁻²·s⁴·A² |
| Magnetic flux | weber | Wb | V·s | kg·m²·s⁻²·A⁻¹ |
| Magnetic flux density | tesla | T | Wb/m² | kg·s⁻²·A⁻¹ |
| Inductance | henry | H | Wb/A | kg·m²·s⁻²·A⁻² |
| Electric field strength | volt per meter | V/m | N/C | kg·m·s⁻³·A⁻¹ |
Electrical unit symbols derived from personal names use uppercase initial letters.
- • Ohm uses the Greek capital omega symbol Ω. Unit names such as volt, ampere, watt, and ohm are lowercase when written out in ordinary text.
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SI Radiation, Photometry and Chemistry Units
Special unit names help distinguish quantities that can have the same algebraic base-unit form.
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| Quantity | Unit | Symbol | SI relation | Important distinction |
|---|---|---|---|---|
| Radionuclide activity | becquerel | Bq | s⁻¹ | Counts nuclear transformations per second |
| Absorbed dose | gray | Gy | J/kg | Energy imparted per unit mass |
| Dose equivalent | sievert | Sv | J/kg | Same dimension as gray, different radiological quantity |
| Luminous intensity | candela | cd | Base unit | Directional photometric quantity |
| Luminous flux | lumen | lm | cd·sr | Photometric flux |
| Illuminance | lux | lx | lm/m² | Luminous flux per area |
| Amount of substance | mole | mol | Base unit | Counts specified entities through amount of substance |
| Catalytic activity | katal | kat | mol/s | Amount converted per unit time |
Special names should be used when they clarify the physical quantity being measured.
- • Bq and Hz both reduce to s⁻¹, but Bq is used for radionuclide activity while Hz is used for frequency. Gy and Sv both reduce to J/kg but represent different dose quantities.
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Identical dimensions do not mean identical physical quantities
Hertz and becquerel both reduce to s⁻¹, but hertz names frequency while becquerel names radionuclide activity. Gray and sievert both reduce to J/kg, yet they identify different dose quantities. Special unit names preserve that physical meaning.
SI Prefix Overview from 10³⁰ to 10⁻³⁰
The current SI recognizes 24 decimal prefixes. Prefix symbols multiply the unit by an exact power of ten.
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| Factor | Prefix | Symbol | Factor | Prefix | Symbol |
|---|---|---|---|---|---|
| 10³⁰ | quetta | Q | 10⁻¹ | deci | d |
| 10²⁷ | ronna | R | 10⁻² | centi | c |
| 10²⁴ | yotta | Y | 10⁻³ | milli | m |
| 10²¹ | zetta | Z | 10⁻⁶ | micro | µ |
| 10¹⁸ | exa | E | 10⁻⁹ | nano | n |
| 10¹⁵ | peta | P | 10⁻¹² | pico | p |
| 10¹² | tera | T | 10⁻¹⁵ | femto | f |
| 10⁹ | giga | G | 10⁻¹⁸ | atto | a |
| 10⁶ | mega | M | 10⁻²¹ | zepto | z |
| 10³ | kilo | k | 10⁻²⁴ | yocto | y |
| 10² | hecto | h | 10⁻²⁷ | ronto | r |
| 10¹ | deca | da | 10⁻³⁰ | quecto | q |
SI prefix factors are exact powers of ten.
- • Prefix symbols are case-sensitive. The four newest prefixes—quetta, ronna, ronto, and quecto—were adopted in 2022.
- • For a full conversion-focused treatment, use the Metric Prefix Chart.
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Prefixes scale units by powers of ten
The SI currently recognizes 24 prefixes from quetta (10³⁰) to quecto (10⁻³⁰). A prefix becomes part of the unit: 1 km is 10³ m and 1 nm is 10⁻⁹ m.
The kilogram is the historical exception. Because kg already contains kilo, mass multiples and submultiples attach the prefix to gram: mg, µg, Mg, and so on. The dedicated Metric Prefix Chart covers the full conversion workflow.
SI Unit Writing Rules Chart
Correct SI notation preserves meaning. Capitalization, spacing, plurals, and prefixes are part of the measurement language.
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| Rule | Correct example | Incorrect example | Why it matters |
|---|---|---|---|
| Leave a space between number and unit symbol | 25 kg | 25kg | The number and unit symbol form separate parts of a quantity value. |
| Do not pluralize unit symbols | 5 kg | 5 kgs | Symbols are mathematical entities, not abbreviations. |
| Do not add a period after a symbol | 10 m | 10 m. | A period belongs only to sentence punctuation. |
| Use lowercase unit names in ordinary text | newton, pascal, watt | Newton, Pascal, Watt | Names derived from people are still lowercase as unit names. |
| Capitalize personal-name unit symbols | N, Pa, W, V | n, pa, w, v | Case can change meaning or make a symbol invalid. |
| Kelvin has no degree sign | 300 K | 300 °K | The SI base unit is kelvin, symbol K. |
| Degree Celsius keeps a space | 20 °C | 20°C | The unit symbol is °C and follows the normal space rule. |
| Use a leading zero below one | 0.25 m | .25 m | Improves readability and reduces decimal-point errors. |
| Use only one prefix | 1 nm | 1 mµm | Compound prefixes are not permitted. |
| Attach mass prefixes to gram | 1 mg | 1 µkg | Kilogram already contains the prefix kilo. |
| Respect symbol case | 1 mW versus 1 MW | Treating mW and MW as equal | Milliwatt and megawatt differ by 10⁹. |
| Use multiplication dots or spaces in compound units | N·m or N m | Nm when ambiguity exists | Clear grouping prevents misreading symbols. |
Examples use common SI writing conventions.
- • NIST U.S. guidance uses meter, liter, and deka, while BIPM international English uses metre, litre, and deca. Unit symbols remain the same.
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Correct SI writing prevents enormous errors
The NIST writing guide emphasizes that symbols are not ordinary abbreviations. They are not pluralized, they do not take periods, and capitalization matters.
Write 5 kg, not 5 kgs. Write 300 K, not 300 °K. Write 20 °C with a space. A milliwatt (mW) and megawatt (MW) differ by a factor of one billion.
Common Non-SI Units Used with SI Measurements
These familiar units are not SI units. Conversions show how they relate to SI quantities; the 2026 SI Brochure clarifies that non-SI units do not hold special status within the SI.
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| Unit | Symbol | SI relationship | Quantity | Note |
|---|---|---|---|---|
| minute | min | 1 min = 60 s | time | Widely used time unit outside the SI |
| hour | h | 1 h = 3,600 s | time | Widely used time unit outside the SI |
| day | d | 1 d = 86,400 s | time | Widely used time unit outside the SI |
| degree of arc | ° | 1° = π/180 rad | plane angle | Common angular unit |
| liter | L | 1 L = 10⁻³ m³ | volume | Decimal submultiple relationship to cubic meter |
| tonne | t | 1 t = 1,000 kg | mass | Decimal multiple of kilogram |
| electronvolt | eV | 1 eV = 1.602176634 × 10⁻¹⁹ J | energy | Exact through the fixed elementary charge |
| dalton | Da | 1.66053906892(52) × 10⁻²⁷ kg | mass | Current value listed in the 2026 SI Brochure |
| nautical mile | — | 1 nautical mile = 1,852 m | length | The 2026 SI Brochure lists no unit symbol |
| knot | — | 1 nautical mile per hour = 1,852/3,600 m/s | speed | The 2026 SI Brochure lists no unit symbol |
Conversions identify SI equivalents; listing a unit here does not make it an SI unit.
- • The SI Brochure version 4.01, published in June 2026, revised the description of non-SI units and explicitly clarified that they do not have special status within the SI.
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How to choose and verify an SI unit
1. Identify the quantity
Decide whether you are measuring length, mass, pressure, energy, concentration, electric potential, dose, or another defined quantity.
2. Start from the coherent SI unit
Use the base unit or coherent derived unit first: m for length, Pa for pressure, J for energy, V for potential difference, and so on.
3. Check the base-unit expression
Dimensional analysis can catch unit mistakes. For example, pressure must reduce to kg·m⁻¹·s⁻².
4. Apply a prefix only when useful
Choose kPa instead of Pa or nm instead of m when it makes the numerical value easier to read without hiding precision.
5. Preserve powers and compound units
A prefix inside cm² is squared with the unit: 1 cm² = 10⁻⁴ m², not 10⁻² m².
6. Keep non-SI units labeled
A minute, liter, tonne, electronvolt, or degree can be converted to SI, but the conversion does not make the original unit an SI unit.
The June 2026 SI Brochure revision clarified its presentation of non-SI units: their appearance in SI documentation does not give them special status within the SI. The SI itself remains the internationally agreed system of base and derived units.
SI units FAQ
What are the seven SI base units?
The seven SI base units are the meter (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), and candela (cd).
How many SI derived units have special names?
There are 22 coherent SI derived units with special names and symbols, including hertz, newton, pascal, joule, watt, volt, tesla, gray, sievert, and katal.
Is the liter an SI unit?
No. The liter is not an SI unit; 1 L equals exactly 10⁻³ m³.
Is Celsius an SI unit?
Degree Celsius, symbol °C, is an SI derived unit with a special name. A temperature interval of 1 °C has the same size as 1 K, while Celsius temperature uses an offset of 273.15 from kelvin.
Is kelvin written with a degree sign?
No. Write kelvin as K, such as 300 K, not °K.
Why is kilogram a base unit even though it contains kilo?
Kilogram is the SI base unit of mass for historical reasons. Decimal multiples and submultiples of mass are formed by attaching prefixes to gram, such as mg and Mg.
Are SI unit symbols plural?
No. Unit symbols never take plural endings, so write 5 kg rather than 5 kgs.
Do you put a space between a number and an SI unit?
Yes. Write a space between the numerical value and the unit symbol, for example 25 m, 20 °C, and 5 kg.
What is a coherent SI unit?
A coherent SI unit is a derived unit obtained from the base units without an extra numerical factor. For example, 1 N equals 1 kg·m·s⁻² exactly.
Is hertz the same as becquerel?
Both Hz and Bq reduce algebraically to s⁻¹, but they name different quantities: hertz is frequency, while becquerel is radionuclide activity.
Are gray and sievert the same unit?
Gray and sievert both reduce to J/kg, but they represent different quantities. Gray measures absorbed dose, while sievert is used for dose-equivalent quantities.
How many SI prefixes are there?
The SI currently recognizes 24 prefixes, ranging from quetta at 10³⁰ to quecto at 10⁻³⁰.
What is the SI unit of force?
The SI unit of force is the newton, symbol N, equal to kg·m·s⁻².
What is the SI unit of pressure?
The SI unit of pressure is the pascal, symbol Pa, equal to one newton per square meter.
What is the SI unit of energy?
The SI unit of energy, work, and amount of heat is the joule, symbol J, equal to N·m or kg·m²·s⁻².
Are all metric units SI units?
No. SI is the internationally defined system of base units, derived units, and prefixes. Some units commonly called metric, such as the liter and tonne, are not SI units even though they have exact decimal relationships to SI units.
Sources
Primary references used for SI definitions, unit relationships, prefixes, and writing conventions:
Bureau International des Poids et Mesures — SI Brochure, 9th edition, version 4.01
Authoritative description of the International System of Units; the 9th edition was updated in June 2026 as version 4.01.
https://www.bipm.org/en/publications/si-brochure
National Institute of Standards and Technology — SI Units
Summarizes the seven SI base units, 22 derived units with special names and symbols, defining constants, prefixes, and SI structure.
https://www.nist.gov/pml/owm/metric-si/si-units
National Institute of Standards and Technology — Writing with SI (Metric System) Units
Provides practical rules for capitalization, plurals, spacing, prefixes, symbols, and the kilogram/gram prefix exception.
https://www.nist.gov/pml/owm/writing-si-metric-system-units