Metric System Basics: Units and Prefixes
The International System of Units (SI) is the modern form of the metric system. It provides defined units, decimal prefixes, and writing rules so measurements can be understood and reproduced internationally.
The seven SI base units
| Quantity | Unit | Symbol |
|---|---|---|
| Time | second | s |
| Length | metre | m |
| Mass | kilogram | kg |
| Electric current | ampere | A |
| Thermodynamic temperature | kelvin | K |
| Amount of substance | mole | mol |
| Luminous intensity | candela | cd |
Derived units combine base units. Speed uses metres per second (m/s); force uses the newton (N), equal to kg·m/s²; and energy uses the joule (J), equal to kg·m²/s².
Decimal prefixes
Prefixes scale a unit by a power of ten. The prefix and unit form one symbol without a space.
| Prefix | Symbol | Factor | Length example |
|---|---|---|---|
| kilo | k | 10³ | 1 km = 1,000 m |
| centi | c | 10⁻² | 1 cm = 0.01 m |
| milli | m | 10⁻³ | 1 mm = 0.001 m |
| micro | µ | 10⁻⁶ | 1 µm = 0.000001 m |
| nano | n | 10⁻⁹ | 1 nm = 0.000000001 m |
How to convert metric lengths
Convert by multiplying by the ratio between prefix factors. For example:
- 2.35 m to cm: 2.35 × 100 = 235 cm.
- 48 mm to cm: 48 ÷ 10 = 4.8 cm.
- 0.72 km to m: 0.72 × 1,000 = 720 m.
Area and volume require squared or cubed conversion factors. Because 1 m = 100 cm, 1 m² = 10,000 cm² and 1 m³ = 1,000,000 cm³.
Notation rules that prevent mistakes
- Leave a space between the value and symbol: 25 mm, not 25mm.
- Do not pluralize symbols: 5 kg, not 5 kgs.
- Symbols are case-sensitive: m means metre, while M means mega.
- The kilogram already contains a prefix; use milligram (mg), not microkilogram.
- Use a leading zero for values below one: 0.5 m, not .5 m.
Using metric units on a ruler
On a standard metric ruler, 10 millimetres equal 1 centimetre. Place the object at the zero mark, read the last whole centimetre, then count additional millimetres. The ruler-marking guide includes worked examples, while the homepage tool requires screen calibration before it represents physical size.