A decimeter (dm) is a unit of length in the International System of Units. It is also referred to as a decimetre outside of the US.

It is based on the meter, the SI base unit of length, where the prefix "deci-" indicates its size of 10-1 relative to the meter. In other words:

1 decimeter = 0.1 meters

Below are a few other commonly used units of length relative to the decimeter.

It is important to choose an appropriate unit of measurement for the given application. For example, if someone were measuring the diameter of a plate, the choice of decimeters as the unit of length would be reasonable. The result would likely be some number in the 10-30 dm range. Measuring the plate in megameters (103) however, would not be a particularly reasonable choice, since 10 dm = 0.000001 Mm. The measurement 0.000001 Mm would be difficult for most people to conceptualize, and would therefore not be particularly useful.

The ability to express various units of measurement in different magnitudes (powers of ten) simply by changing the prefix on the base unit is a major advantage of SI compared to many other systems of measurement.

SI units and prefixes

SI prefixes, such as "deci-" are added to an SI unit to indicate a multiple or submultiple of the unit. In either case, the units will be related by some power of ten. This makes it very easy to pick a unit that is appropriate in magnitude when using SI. Doing so only requires us to learn (or reference) the various SI prefixes.

In contrast, while it is also possible to pick measurements of length that are appropriate for the given situation in the US customary system of units, it is also more tedious, since converting between these units requires knowledge of the conversion factors. In SI, all units of measurement are related by some power of 10, so it is only necessary to shift the decimal place. In most other systems of measurement, various units are used to measure the same attribute, and these units typically are not related by a constant power, as they are in SI.

In the US customary system of measurement for example, inches, feet, yards, and miles are commonly used units of length, all of which are related by different factors. There are 12 inches in 1 foot, 3 feet in 1 yard, and 1760 yards in 1 mile. Even knowing these conversion factors, it is still more tedious to convert between the various lengths than simply counting decimal places in SI. Also, none of these conversion factors apply to other units of measurement in the US customary system, while SI prefixes can be applied to any SI base unit.