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.
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 



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 casesensitive, so uppercase and lowercase letters have different meanings — for example, mm is the millimeter (onethousandth 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
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 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 
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 
poundforce (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 avordupois units.
Note 2. Fluid ounces, quarts, and gallons refer to US liquid measures.
