The International System of Units (SI)
The 7 SI Base Units
The seven SI base units are:
- metre (m) — length
- kilogram (kg) — mass
- second (s) — time
- ampere (A) — electric current
- kelvin (K) — thermodynamic temperature
- mole (mol) — amount of substance
- candela (cd) — luminous intensity
The SI is the globally agreed metric system adopted by the CGPM. Since the 2019 redefinition, the base units are defined by fixed values of fundamental constants
(e.g., h, e, k, NA) for long-term stability and universal realization.
The 7 SI base units with names and symbols
| SI base units |
| Base quantity |
Unit name |
Symbol |
| length | metre | m |
| mass | kilogram | kg |
| time | second | s |
| electric current | ampere | A |
| thermodynamic temperature | kelvin | K |
| amount of substance | mole | mol |
| luminous intensity | candela | cd |
Common SI derived units with special names and base-unit expressions
| SI derived units (selected) |
| Derived quantity |
Special name |
Symbol |
In base units |
| plane angle | radian | rad | rad = m/m = 1 |
| solid angle | steradian | sr | sr = m2/m2 = 1 |
| frequency | hertz | Hz | Hz = s−1 |
| force | newton | N | N = kg·m·s−2 |
| pressure, stress | pascal | Pa | Pa = N·m−2 = kg·m−1·s−2 |
| energy, work | joule | J | J = N·m = kg·m2·s−2 |
| power | watt | W | W = J·s−1 = kg·m2·s−3 |
| electric charge | coulomb | C | C = A·s |
| electric potential | volt | V | V = W·A−1 = kg·m2·s−3·A−1 |
| capacitance | farad | F | F = C·V−1 = kg−1·m−2·s4·A2 |
| resistance | ohm | Ω | Ω = V·A−1 = kg·m2·s−3·A−2 |
| conductance | siemens | S | S = Ω−1 = kg−1·m−2·s3·A2 |
| magnetic flux | weber | Wb | Wb = V·s = kg·m2·s−2·A−1 |
| magnetic flux density | tesla | T | T = Wb·m−2 = kg·s−2·A−1 |
| inductance | henry | H | H = Wb·A−1 = kg·m2·s−2·A−2 |
| Celsius temperature | degree Celsius | °C | Δ°C = ΔK |
| luminous flux | lumen | lm | lm = cd·sr |
| illuminance | lux | lx | lx = lm·m−2 |
Writing SI Correctly
- Use a space between number and unit:
25 °C, 10 N·m (except for plane/solid angle symbols).
- Unit symbols are case-sensitive and do not take a period:
m, kg, Pa, °C.
- Use · (middle dot) or a space for multiplication and negative exponents for division:
N·m, kg·m·s−2.
- Radian (
rad) and steradian (sr) are dimensionless derived units with special names.
2019 SI redefinition: The base units are tied to exact values of fundamental constants (Planck constant h, elementary charge e, Boltzmann constant k, Avogadro constant NA), making SI realizations device-independent.
Supplements
Reference
- ISO 80000-1 — Quantities and Units: General