# What determines whether a numerical prefix is allowable for a unit? [closed]

1. The prefix centi- means 10-2. But while centimetre is common, I have never heard of a centiwatt or a centisecond.
2. The prefix Mega- means 106. Hence Megabytes exist. So why do we not have Megametres?
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## closed as general reference by Daniel, MrHen, Jasper Loy, kiamlaluno, waiwai933♦Jul 24 '11 at 17:17

This question is too basic; it can be definitively and permanently answered by a single link to a standard internet reference source designed specifically to find that type of information.If this question can be reworded to fit the rules in the help center, please edit the question.

If the unit is a metric unit, then all metric prefixes are allowable.

1. The centiwatt and centisecond are actual units of measurement, albeit rare, since one would tend to say 10 milliwatts or milliseconds instead.

2. We do have megametres.

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But why are some used, while others are not? –  Urbycoz Jul 22 '11 at 12:54
Well, seriously, who measures by millions of meters? Also, all those terms are used, but certain ones, like centiwatts are only used in highly specialized situations. (When was the last time you worked with centiwatts?) –  Daniel Jul 22 '11 at 12:59
@drm65 Astronomers do. –  KitFox Jul 22 '11 at 13:00
@Kit: I knew that was coming - though to an astronomer, a thousand kilometers is a very slight distance. Even astronomers say "a thousand kilometers" rather than "a megameter", in most cases. Cf. the wikipedia page I referenced for megameter. –  Daniel Jul 22 '11 at 13:03
People generally use milliwatts rather than centiwatts. It's a matter of custom, and not determined by a central authority. More often than not, it depends on what unit is the proper size for a given application; this is why people often use Angstroms rather than nanometers. –  Peter Shor Jul 22 '11 at 13:05

For scientists and engineers, the prefixes are defined by the International System of Units (SI). The exponents increase in steps of three.

(There is a link on the SI units page to IEC units used for computers)

Older MKS units like centi are no longer preferred.

[If you see a reference in the Jargon File to proposed units harpo- and groucho- remember, they were a joke.]

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