# 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?
• Except a megabyte is not a million bytes. – kinokijuf Jan 6 '14 at 8:18

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.

• 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. – Kit Z. Fox 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

All of the things you have mentioned exist, for exactly the reason you expect they should. If some are more common than others, it's likely because (a) only a few orders of magnitude apply to most kinds of measurement in everyday life and (b) most people are not scientists and, therefore, would rather say "a thousand kilometres" than "a megameter"... possibly because they don't even know what "mega" means.

By the by, isn't a megabyte equal to 1024 kilobytes (2^20 bytes), not 1000 kilobytes (10^6)? hah.

• Nowadays, a megabyte in storage devices is equal to a million bytes. :( – Daniel Jul 22 '11 at 13:19
• According to whom? People who make hard drives? I could believe that. – Patrick87 Jul 22 '11 at 13:23
• Manufacturers of disk drives have hijacked the the prefix mega to mean 1,000,000 and we have to use mebibytes to mean 1,048,576 = 2^20 (en.wikipedia.org/wiki/Mebibyte) – pavium Jul 22 '11 at 13:24
• Yes, and now we are left to differentiate between the storage device GBs and GBs according to the computer. Buy a TB hard drive, stick it in the computer, and it says you have 931 GB. – Daniel Jul 22 '11 at 13:25
• Whatever. I'll keep using "mega" as God intended, and if a Mars rover has to go down because of it, I'll still sleep easy at night. – Patrick87 Jul 22 '11 at 13:26

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.]