SI Units Standards Explained
Understanding the International System of Units—the foundation of scientific measurement.
Why & What
The International System of Units (SI, from French: Système International d'Unités) is the modern form of the metric system. It's the world's most widely used system of measurement, used in science, industry, and commerce globally.
SI provides a standardized framework that allows scientists, engineers, and ordinary people to communicate measurements unambiguously across languages and borders.
The Seven SI Base Units
All SI units can be derived from these seven fundamental units:
| Quantity | Unit Name | Symbol | Definition |
|---|---|---|---|
| Length | meter | m | Distance light travels in 1/299,792,458 second |
| Mass | kilogram | kg | Defined by the Planck constant |
| Time | second | s | 9,192,631,770 periods of cesium-133 |
| Electric Current | ampere | A | Flow of elementary charges per second |
| Temperature | kelvin | K | Defined by the Boltzmann constant |
| Amount of Substance | mole | mol | 6.02214076 × 10²³ entities |
| Luminous Intensity | candela | cd | Luminous power per steradian |
Common Derived Units
These units are derived from the seven base units:
Force: kg⋅m/s²
Energy: kg⋅m²/s²
Power: J/s = kg⋅m²/s³
Pressure: N/m² = kg/(m⋅s²)
Voltage: W/A = kg⋅m²/(A⋅s³)
Frequency: 1/s
SI Prefixes
Prefixes are used to create multiples and submultiples of SI units:
| Larger | Symbol | Factor |
|---|---|---|
| tera | T | 10¹² |
| giga | G | 10⁹ |
| mega | M | 10⁶ |
| kilo | k | 10³ |
| hecto | h | 10² |
| Smaller | Symbol | Factor |
|---|---|---|
| deci | d | 10⁻¹ |
| centi | c | 10⁻² |
| milli | m | 10⁻³ |
| micro | μ | 10⁻⁶ |
| nano | n | 10⁻⁹ |
Metric vs Imperial
While SI (metric) is used globally in science, the Imperial system is still common in the United States, United Kingdom (partially), and a few other countries.
- Metric: Base-10, coherent, used worldwide in science
- Imperial: Historical units, non-decimal relationships
- NASA's Mars Climate Orbiter was lost due to a metric-imperial conversion error
Note
- SI definitions were updated in 2019 to be based on fundamental constants.
- Some fields use specialized units (e.g., astronomy uses parsecs, light-years).
- For historical or legal reasons, non-SI units remain in use in some contexts.