1.1.1: SI Prefixes
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- Apr 12, 2022
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Some “derived” units are just a prefix in front of a regular unit. There is a standard set of SI prefixes that can be prepended to any unit in order to make another unit of the same dimensionality but of a different size. The ones you are probably most familiar with are milli and kilo. A millimeter is 1/1000 of a meter, and a kilometer is 1000 meters. You could do the same thing with seconds; a millisecond is 0.001 seconds, and a kilosecond is 1000 seconds (about 17 minutes). Indeed, the SI mass unit, the kilogram, is itself 1000 grams. In this class, we will frequently talk about things that are much smaller, such as nanometers and microseconds. If you are in an astronomy class, you might find yourself using the mega or giga prefixes more often. The table below summarizes the prefixes.
| Prefix | Abbreviation | Multiplier |
|---|---|---|
| terra | T | 1012 |
| giga | G | 109 |
| mega | M | 106 |
| kilo | k | 103 |
| deci | d | 10−1 |
| centi | c | 10−2 |
| milli | m | 10−3 |
| micro | µ | 10−6 |
| nano | n | 10−9 |
| pico | p | 10−12 |
| femto | f | 10−15 |
Notice that case matters. There is very big difference between a Mm and a mm— a factor of a billion, in fact! The letter used to indicate micro is the Greek letter mu. There are a million µs in one second. The prefixes deci and centi are not used very often, and generally only with meters. While you will talk about centimeters, nobody generally talks about centigrams or centiseconds.
