What are prefixes for?

Why use prefixes?

In physics, we measure very different quantities: the Earth-Moon distance (very large) or the thickness of a hair (very small). Writing these numbers using only meters would give enormous or tiny figures, hard to read.

Prefixes are words (or letters) added in front of a unit to indicate that it is multiplied or divided by a power of 10. They make it possible to write measurements in a simpler and more readable way.

The most common prefixes

Prefix	Symbol	Factor	Example
kilo	k	x 1000	1 km = 1000 m
hecto	h	x 100	1 hm = 100 m
deca	da	x 10	1 dam = 10 m
(unit)	m, g, s...	x 1	1 m
deci	d	/ 10	1 dm = 0.1 m
centi	c	/ 100	1 cm = 0.01 m
milli	m	/ 1000	1 mm = 0.001 m

How to read a prefix

A prefix is always read the same way, whatever the base unit: kilo means "a thousand times bigger", milli means "a thousand times smaller". Thus:

• 1 kg = 1000 g (kilogram)
• 1 km = 1000 m (kilometer)
• 1 ms = 0.001 s (millisecond)

Common pitfall

Don't confuse the symbol "m" for meter with the prefix "m" for milli! In "mm" (millimeter), the first m is the prefix milli, the second is the unit meter. Always look at the context to correctly identify the base unit.