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 |
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. |
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
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.
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.
