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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

Quantity measuredUnitSymbolRelationship
Length, width,
distance, thickness,
girth, etc.
millimetermm10 mm=1 cm
centimetercm100 cm=1 m
kilometerkm1 km=1000 m
milligrammg1000 mg=1 g
kilogramkg1 kg=1000 g
metric tont1 t=1000 kg
Temperaturedegree Celsius°C   
Areasquare meter   
hectareha1 ha=10 000 m²
square kilometerkm²1 km²=100 ha
VolumemillilitermL1000 mL=1 L
cubic centimetercm³1 cm³=1 mL
literL1000 L=1 m³
cubic meter   
Speed, velocitymeter per secondm/s   
kilometer per hourkm/h1 km/h=0.278 m/s
Densitykilogram per cubic meterkg/m³   
Pressure, stresskilopascalkPa   
kilowattkW1 kW=1000 W
megajouleMJ1 MJ=1000 kJ
kilowatt hourkW·h1 kW·h=3.6 MJ
Electric currentampereA   

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

The most commonly used metric 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.

gigaG1091 000 000 000billion
megaM1061 000 000million
kilok1031 000thousand
microμ10-60.000 001millionth
nanon10-90.000 000 001billionth

A note about usage

Although unit names are ordinary words, note that unit symbols

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

Some examples and relationships among units

Metric ton

1 liter of water weighs 1 kilogram, so 1 cubic meter — 1000 liters — of water weighs 1000 kilograms or 1 metric ton.


A US cent weighs exactly 2.5 g, while the nickel weighs exactly 5 g.


A doorknob is typically about 1 m high.


The diameter of a CD or DVD is 12 cm.

1 mL = 1 cm3

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 m3 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 1002 m2

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.

Approximate conversion factors from inch-pound to metric units

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 fromtomultiply byMore precisely,
multiply by
acres (US survey)hectares (ha)0.40.404 687 3 
feet (ft)meters (m)0.30.3048 
fluid ounces (fl oz)milliliters (mL)3029.573 532
gallons (gal)liters (L)3.83.785 411 7842
inches (in)centimeters (cm)2.542.54 
knotskilometers per hour (km/h) 1.852 
miles (mi)kilometers (km)1.61.609 344 
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)1.61.609 344 
nautical mileskilometers 1.852 
ounces (oz)grams (g)2828.349 521
pound-force (lbf)newtons (N) 4.448 222 
pounds (lb)kilograms (kg)0.45
or divide by 2.2
0.453 592 371
pounds per square inch (lbf/in2)kilopascals (kPa) 6.894 757 
quarts (qt)liters (L)0.90.946 352 9462
square feet (ft2)square meters (m2)0.10.092 903 04 
square miles (mi2)square kilometers (km2)2.62.589 988 
yards (yd)meters (m)0.90.9144 

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: 2014-08-05