Foundation
The seven base quantities and units
A physical quantity and its unit are different concepts. Quantity symbols are commonly italic; unit symbols are upright and never pluralized.
| Base quantity | Typical quantity symbol | SI base unit | Unit symbol |
|---|---|---|---|
| time | t | second | s |
| length | l, x, r | metre | m |
| mass | m | kilogram | kg |
| electric current | I | ampere | A |
| thermodynamic temperature | T | kelvin | K |
| amount of substance | n | mole | mol |
| luminous intensity | Iᵥ | candela | cd |
The kilogram is the only base unit whose name already contains a prefix. Prefixes for mass attach to gram: 1 mg = 10⁻⁶ kg, not 10⁻³ kg.
Combinations
Common derived SI units
Derived units are coherent products of powers of the base units. Some receive special names and symbols for clarity.
| Quantity | Unit | Symbol | In base units |
|---|---|---|---|
| plane angle | radian | rad | m/m = 1 |
| frequency | hertz | Hz | s⁻¹ |
| force | newton | N | kg·m·s⁻² |
| pressure | pascal | Pa | kg·m⁻¹·s⁻² |
| energy | joule | J | kg·m²·s⁻² |
| power | watt | W | kg·m²·s⁻³ |
| electric charge | coulomb | C | A·s |
| voltage | volt | V | kg·m²·s⁻³·A⁻¹ |
| capacitance | farad | F | kg⁻¹·m⁻²·s⁴·A² |
| resistance | ohm | Ω | kg·m²·s⁻³·A⁻² |
| magnetic flux | weber | Wb | kg·m²·s⁻²·A⁻¹ |
| magnetic flux density | tesla | T | kg·s⁻²·A⁻¹ |
| inductance | henry | H | kg·m²·s⁻²·A⁻² |
| luminous flux | lumen | lm | cd·sr |
| illuminance | lux | lx | lm·m⁻² |
| radioactivity | becquerel | Bq | s⁻¹ |
| absorbed dose | gray | Gy | m²·s⁻² |
| dose equivalent | sievert | Sv | m²·s⁻² |
| catalytic activity | katal | kat | mol·s⁻¹ |
Scale
SI prefixes: quecto to quetta
A prefix multiplies the unit by one power of ten. Prefix symbols are case-sensitive: mW is a milliwatt, while MW is a megawatt.
10³⁰10²⁷10²⁴10²¹10¹⁸10¹⁵10¹²10⁹10⁶10³10²10¹10⁻¹10⁻²10⁻³10⁻⁶10⁻⁹10⁻¹²10⁻¹⁵10⁻¹⁸10⁻²¹10⁻²⁴10⁻²⁷10⁻³⁰Interactive prefix conversion
Worked checks
Conversion examples
3.2 × 10³ m = 3200 m
450 × 10⁻⁹ m = 4.50 × 10⁻⁷ m
2.5 × 10⁻⁶ F
(10⁻² m)³ = 10⁻⁶ m³
Notation
Common SI mistakes
- Write
25 °C,10 m, and5 kgwith a space between value and unit symbol. - Use
K, not°K. The degree sign belongs to degrees Celsius,°C. - Unit symbols do not take plural forms or periods:
5 kg, not “5 kgs”. - Do not combine prefixes: use
nm, not “mµm”. - When a prefixed unit is squared or cubed, the prefix is also raised to that power.
- Use uppercase and lowercase exactly:
Pa,mPa, andMPaare different.
Primary references
BIPM — The International System of Units (SI Brochure)BIPM — SI prefixesReviewed August 11, 2026 · STEM Hub review methodology
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