## Commonly used metric system units, symbols, and prefixes |

**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.**

- Commonly used metric system units and symbols
- The most commonly used metric prefixes
- A note about usage
- Some examples and relationships among units
- Approximate conversion factors from inch-pound to metric units

Quantity measured | Unit | Symbol | Relationship | ||
---|---|---|---|---|---|

Length, width, distance, thickness, girth, etc. | millimeter | mm | 10 mm | = | 1 cm |

centimeter | cm | 100 cm | = | 1 m | |

meter | m | ||||

kilometer | km | 1 km | = | 1000 m | |

Mass (“weight”)* | milligram | mg | 1000 mg | = | 1 g |

gram | g | ||||

kilogram | kg | 1 kg | = | 1000 g | |

metric ton | t | 1 t | = | 1000 kg | |

Time | second | s | |||

Temperature | degree Celsius | °C | |||

Area | square meter | m² | |||

hectare | ha | 1 ha | = | 10 000 m² | |

square kilometer | km² | 1 km² | = | 100 ha | |

Volume | milliliter | mL | 1000 mL | = | 1 L |

cubic centimeter | cm³ | 1 cm³ | = | 1 mL | |

liter | L | 1000 L | = | 1 m³ | |

cubic meter | m³ | ||||

Speed, velocity | meter per second | m/s | |||

kilometer per hour | km/h | 1 km/h | = | 0.278 m/s | |

Density | kilogram per cubic meter | 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 |

* See *Is it “weight”
or “mass”?* in the FAQ.

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 | Numerically | Name |
---|---|---|---|---|

giga | G | 10^{9} | 1 000 000 000 | billion** |

mega | M | 10^{6} | 1 000 000 | million |

kilo | k | 10^{3} | 1 000 | thousand |

centi | c | 10^{-2} | 0.01 | hundredth |

milli | m | 10^{-3} | 0.001 | thousandth |

micro | μ | 10^{-6} | 0.000 001 | millionth |

nano | n | 10^{-9} | 0.000 000 001 | billionth** |

** | The terms billion,
trillion, etc., can be ambiguous. The terms are used here
in the American English sense, but British English traditionally
defined billion as a million million, rather than a
thousand million. However, recent British usage tends to match
the American meanings. |

Although unit names are ordinary words, note that unit symbols

are case-sensitive, so uppercase and lowercase letters have different meanings — for example, mm is the millimeter (one-thousandth of a meter), but Mm is the megameter (one million meters);

don't have singular and plural forms — it's 1 mL, 2 mL (no “s” at the end); and

aren't abbreviations, so there's no period after a unit symbol (unless it happens to fall at the end of a sentence).

For more details on usage, including some common errors, read the USMA's page on correct SI-metric usage. In addition, the FAQ includes some information on usage.

1 mL = 1 cm^{3}

1 milliliter is the same volume as 1 cubic centimeter.

1 mL of water has a mass of approximately 1 g

The mass of 1 milliliter of water is approximately 1 gram.

1 L of water has a mass of approximately 1 kg

The mass of 1 liter of water is therefore approximately 1 kilogram.

1 m^{3} of water has a mass of approximately 1 t

There are 1000 liters in a cubic meter, so the mass of 1 cubic meter of water is approximately 1000 kilograms or 1 metric ton.

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 meter above the floor.

The diameter of a CD or DVD is 12 cm

A CD or DVD is 12 centimeters (120 millimeters) across. The diameter of the center hole is 15 millimeters.

1 ha is 100^{2} m^{2}

1 hectare is 10 000 square meters, equivalent to the area of a square 100 meters on a side. A football field is about 100 meters long, so imagine a square the length of a football field on each side, and that's 1 hectare.

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 meters
away. Only if it was *exactly* 100 yards away would one
convert the measurement to 91.44 meters.

To convert from | to | multiply by | More precisely, multiply by | Note |
---|---|---|---|---|

acres (US survey) | hectares (ha) | 0.4 | 0.404 687 3 | |

feet (ft) | meters (m) | 0.3 | 0.3048 | |

fluid ounces (fl oz) | milliliters (mL) | 30 | 29.573 53 | 2 |

gallons (gal) | liters (L) | 3.8 | 3.785 411 784 | 2 |

inches (in) | centimeters (cm) | 2.54 | 2.54 | |

knots | kilometers per hour (km/h) | 1.852 | ||

miles (mi) | kilometers (km) | 1.6 | 1.609 344 | |

miles per gallon (mi/gal) | liters per 100 km (L/(100 km)) | divide 235.215 by mi/gal | ||

miles per hour (mi/h) | kilometers per hour (km/h) | 1.6 | 1.609 344 | |

nautical miles | kilometers | 1.852 | ||

ounces (oz) | grams (g) | 28 | 28.349 52 | 1 |

pound-force (lbf) | newtons (N) | 4.448 222 | ||

pounds (lb) | kilograms (kg) | 0.45 or divide by 2.2 | 0.453 592 37 | 1 |

pounds per square inch (lbf/in^{2}) | kilopascals (kPa) | 6.894 757 | ||

quarts (qt) | liters (L) | 0.9 | 0.946 352 946 | 2 |

square feet (ft^{2}) | square meters (m^{2}) | 0.1 | 0.092 903 04 | |

square miles (mi^{2}) | square kilometers (km^{2}) | 2.6 | 2.589 988 | |

yards (yd) | meters (m) | 0.9 | 0.9144 |

**Note 1.** Ounces and pounds refer to avoirdupois units.

**Note 2.** Fluid ounces, quarts, and gallons refer to US liquid measures.

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Updated: 2007-03-08