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SI Units & Prefixes
The seven SI base units, common derived units, and metric prefixes in one reference table.
A complete reference of SI base units, derived units, and metric prefixes — scroll the tables below.
📏The seven SI base units
The International System of Units (SI) is built on seven base units. Every other unit in science is derived from these.
| Quantity | Unit | Symbol |
|---|---|---|
| Length | metre | m |
| Mass | kilogram | kg |
| Time | second | s |
| Electric current | ampere | A |
| Temperature | kelvin | K |
| Amount of substance | mole | mol |
| Luminous intensity | candela | cd |
Common derived units
Derived units are combinations of the base units, often given their own name.
| Quantity | Unit | Symbol | In base units |
|---|---|---|---|
| Force | newton | N | kg·m/s² |
| Energy / Work | joule | J | N·m |
| Power | watt | W | J/s |
| Pressure | pascal | Pa | N/m² |
| Frequency | hertz | Hz | 1/s |
| Electric charge | coulomb | C | A·s |
| Voltage | volt | V | J/C |
| Resistance | ohm | Ω | V/A |
Metric prefixes
Prefixes scale a unit up or down by powers of ten — for example, 1 km = 10³ m.
| Prefix | Symbol | Factor |
|---|---|---|
| giga | G | 10⁹ |
| mega | M | 10⁶ |
| kilo | k | 10³ |
| deci | d | 10⁻¹ |
| centi | c | 10⁻² |
| milli | m | 10⁻³ |
| micro | µ | 10⁻⁶ |
| nano | n | 10⁻⁹ |
| pico | p | 10⁻¹² |
💡 Frequently Asked Questions
Why are SI units important?+
SI units give scientists worldwide a common, unambiguous language for measurements, so results can be compared and reproduced anywhere.
Is the kilogram really a base unit, not the gram?+
Yes — unusually, the SI base unit of mass is the kilogram (kg), not the gram, for historical reasons tied to how the standard was originally defined.