SI Unit Prefixes


Learning Objective

  • Convert between SI units

Key Points

    • The set of prefixes is simple and easy to use.
    • Prefixes cannot be combined.
    • The set of prefixes is universal.


  • prefixOne or more letters or syllables added to the beginning of a word to modify its meaning; for example, kilo can be added to gram to create kilogram
  • fractiona part of a whole, especially a comparatively small part

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.

The basic SI unitsThe basic units of the SI system.

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 unitsThe 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.

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