In the International System of Units (SI), each physical quantity—length, mass, volume, etc.—is represented by a specific SI unit. Larger and smaller multiples of that unit are made by adding SI prefixes. This page shows the most commonly used units, symbols, and prefixes, and some other basic information to start using it as quickly as possible. For more details, see Detailed list of metric system units and symbols and SI prefixes and their etymologies.
Contents
Units and symbols
| Quantity measured | Unit | Symbol | Relationship |
|---|---|---|---|
| Length, width, distance, thickness, girth, etc. |
millimetre | mm | 1000 mm = 1 m |
| metre | m | ||
| kilometre | km | 1 km = 1000 m | |
| Mass (“weight”)* |
milligram | mg | 1000 mg = 1 g |
| gram | g | 1000 g = 1 kg | |
| kilogram | kg | ||
| megagram (tonne or “metric ton”)† | Mg (t) | 1 Mg = 1 t = 1000 kg | |
| Time | second | s | |
| minute | min | 1 min = 60 s | |
| hour | h | 1 h = 60 min | |
| Temperature | degree Celsius | °C | |
| Area | square metre | m2 | 1 m2 = 1 m × 1 m |
| hectare | ha | 1 ha = 10 000 m2 | |
| square kilometre | km2 | 1 km2 = 100 ha | |
| Volume | millilitre | mL | 1000 mL = 1 L |
| litre | L | 1000 L = 1 m3 | |
| cubic metre | m3 | 1 m3 = 1 m × 1 m × 1 m | |
| Speed, velocity | metre per second | m/s | |
| kilometre per hour | km/h | 1 km/h = 0.278 m/s | |
| Density | kilogram per cubic metre | kg/m3 | |
| Force | newton | N | |
| Pressure, stress | kilopascal | kPa | |
| Power | watt | W | |
| kilowatt | kW | 1 kW = 1000 W | |
| Energy | kilojoule | kJ | |
| megajoule | MJ | 1 MJ = 1000 kJ | |
| kilowatt-hour | kW⋅h | 1 kW⋅h = 3.6 MJ | |
| Electric current | ampere | A | |
| Electric charge | coulomb | C | |
| milliampere-hour | mA⋅h | 1 mA⋅h = 3.6 C |
Notes
* See Is it ‘weight’ or ‘mass’? in the FAQ.
† See How about ‘tons’ and ‘tonnes’? in the FAQ for more details.
Prefixes
This table shows the most commonly used SI prefixes. For a complete list of SI prefixes, including their origins, see SI prefixes and their etymologies.
| Prefix | Symbol | Factor | Ordinary Notation | Name | |
|---|---|---|---|---|---|
| giga | G | 109 | 1 000 000 000 | billion | |
| mega | M | 106 | 1 000 000 | million | |
| kilo | k | 103 | 1 000 | thousand | |
| 100 | 1 | one, unity | |||
| milli | m | 10−3 | 0 | .001 | thousandth |
| micro | μ | 10−6 | 0 | .000 001 | millionth |
| nano | n | 10−9 | 0 | .000 000 001 | billionth |
Usage
Although unit names are ordinary words, note that unit symbols are just that (symbols) and not abbreviations. They
- are case-sensitive: uppercase and lowercase letters have different meanings—for example, mm is the millimetre (one-thousandth of a metre), but Mm is the megametre (one million metres);
- don’t have singular and plural forms: it’s 1 km, 2 km (no “s” at the end);
- have spaces before them: it’s 1 km, not 1km; and
- don’t have periods / full stops after them (unless they happen to fall at the ends of sentences).
For more details on usage, including some common errors, read the USMA’s page on correct SI usage. In addition, the FAQ includes some information on usage. For information on typing symbols properly, see How can I type unit symbols such as m2, °C, N⋅m, and µm? in the FAQ.
Examples and relationships
- 1 mL = 1 cm3
- 1 millilitre is the same volume as 1 cubic centimetre.
- 1 mL of water has a mass of approximately 1 g
- The mass of 1 millilitre of water is approximately 1 gram.
- 1 L of water has a mass of approximately 1 kg
- The mass of 1 litre of water is therefore approximately 1 kilogram.
- 1 m3 of water has a mass of approximately 1 Mg
-
There are 1000 litres in a cubic metre, so the mass of 1 cubic metre of water is approximately 1000 kilograms or 1 megagram.
- The mass of a nickel is 5 g
-
A US nickel weighs 5 grams, and a penny weighs 2.5 grams.
- A typical doorknob is 1 m high
-
Although there’s no precise standard for doorknob heights, they’re often about 1 metre above the floor.
- The diameter of a CD or DVD is 120 mm
-
A CD or DVD is 120 millimetres across. The diameter of the center hole is 15 millimetres.
- 1 ha = 10 000 m2 (100 m × 100 m)
- 1 hectare is 10 000 square metres, equivalent to the area of a square 100 metres on each side. A football field is about 100 metres long, so imagine a square the length of a football field on each side, and that’s 1 hectare.
Conversion factors
This table gives easily remembered, approximate conversion factors for some common units, as well as more precise factors. Boldfaced values are exact. But remember, estimated values don’t warrant precise conversions. If “it was about 100 yards away”, then it was about 100 metres away. Only if it was exactly 100 yards away would one convert the measurement to 91.44 metres.
| To convert from | to | multiply by | More precisely, multiply by |
|---|---|---|---|
| acres (US survey) | hectares (ha) | 0.4 | 0.404 687 3 |
| feet (ft) | metres (m) | 0.3 | 0.3048 |
| fluid ounces (fl oz)* | millilitres (mL) | 30 | 29.573 53 |
| gallons (gal)* | litres (L) | 3.8 | 3.785 411 784 |
| inches (in) | millimetres (mm) | 25 | 25.4 |
| knots (kn) | kilometres per hour (km/h) | 1.852 | |
| miles (mi) | kilometres (km) | 1.6 | 1.609 344 |
| miles per gallon (mpg) | litres per 100 km [L/(100 km)] | Divide 235.215 by mpg |
|
| miles per hour (mph) | kilometres per hour (km/h) | 1.6 | 1.609 344 |
| nautical miles (nmi) | kilometres (km) | 1.852 | |
| ounces (oz)† | grams (g) | 28 | 28.349 52 |
| pounds-force (lbf) | newtons (N) | 4.448 222 | |
| pounds (lb)† | kilograms (kg) | 0.45 or divide by 2.2 |
0.453 592 37 |
| pounds per square inch (lbf/in2, psi) | kilopascals (kPa) | 6.894 757 | |
| quarts (qt)* | litres (L) | 0.9 | 0.946 352 946 |
| square feet (sq ft) | square metres (m2) | 0.1 | 0.092 903 04 |
| square miles (sq mi) | square kilometres (km2) | 2.6 | 2.589 988 |
| yards (yd) | metres (m) | 0.9 | 0.9144 |
Notes
* Fluid ounces, quarts, and gallons refer to US liquid measures.
† Ounces and pounds refer to avoirdupois units.
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