The basic SI units can be expressed as fractions and multiples of basic units by using a set of simple prefixes.
SI Unit Prefixes
Now that we know about the SI system and what it provides to the scientist and engineer, we can examine some aspects of actual measurement. The SI system utilizes a standard system of prefixes to the basic units that allow them to be more relevant to and descriptive of relative magnitude.
For example, when reading about chemical kinetics, you may encounter the terms “ms” or “ns,” meaning “millisecond” and “nanosecond” respectively. Once you’ve become accustomed to the practice of using the prefixes, you will immediately know that the millisecond is 1/1000 of one second and is 1 million times larger than a nanosecond, which is 1/1000000000 of one second, or 10-9 seconds.
Briefly review the basic SI units before you study the prefixes.
| Unit name | Unit symbol | Quantity name | Quantity symbol | Dimension symbol |
|---|---|---|---|---|
| meter | m | length | l, x, r | L |
| kilogram | kg | mass | m | M |
| second | s | time | t | T |
| ampere | A | electric current | I | I |
| kelvin | K | thermodynamic temperature | T | Θ |
| candela | cd | luminous intensity | Iv | J |
| mole | mol | amount of substance | n | N |
There are 20 accepted prefixes. A prefix may be used to identify multiples of the original unit or fractions of the original unit. For example, kilo- denotes a multiple of a thousand, so there are one thousand meters in a kilometer. Milli- denotes a thousandth; therefore, there are one thousand millimeters in a meter.
Prefixes for SI units: The prefixes redefine the measurement as either a multiple or a fraction of the basic unit.
Keep in mind that prefixes should never be combined. Thus a millionth of a meter is a micrometer, not a millimillimeter, and a millionth of a kilogram is a milligram, not a microkilogram.
In older usage, a micron (a measurement often encountered in physics and engineering) is the same as a micrometer, 10-6 meters. Another older form of usage, the millimicron, is one thousandth of a micrometer, or 1 thousandth of 10-6 meters, or 10-9 meter, now called a nanometer. While these older terms are not in common usage, they are often encountered in older publications, and knowing their modern equivalents is an advantage.