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Metric system

 

he 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 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      
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      
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      




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.

Prefix Symbol Factor Numerically Name
giga G 109 1 000 000 000 billion**
mega M 106 1 000 000 million
kilo k 103 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.




A note about usage

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).

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.

Coins

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

Doorknob

A doorknob is typically about 1 m high.

DVD

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 from to multiply by More precisely,
multiply by
Note
acres 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/in2) kilopascals (kPa)   6.894 757  
quarts (qt) liters (L) 0.9 0.946 352 946 2
square feet (ft2) square meters (m2) 0.1 0.092 903 04  
square miles (mi2) square kilometers (km2) 2.6 2.589 988  
yards (yd) meters (m) 0.9 0.9144  

Note 1. Ounces and pounds refer to avordupois units.

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