A decameter (dam) is a unit of length in the International System of Units (SI). It is also referred to as a decametre outside of the US.

It is based on the meter, the SI base unit of length, where the prefix "deca-" indicates a factor of ten. In other words:

1 decameter = 10 meters

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

It is important to use measurements of length that are appropriate to the scale and application. For example, a decameter can be reasonably used to measure the height of a tree, building, or waterfall. However, a decameter would not be used to measure the size of an ant, since ants are significantly smaller than a decameter, and the fraction or decimal would be so small that it would be difficult to conceptualize the size of the ant based on the measurement. This is one reason that the use of prefixes that modify base units in SI is so useful.

SI units and prefixes

SI prefixes, such as "deca-" are added to an SI unit to indicate a multiple or submultiple of the unit. There are many different SI prefixes that each denote a power of ten, so it is fairly simple to pick a unit that is appropriate in magnitude when using SI. Using the same example described above, a millimeter or centimeter would be a more appropriate measurement of length for an ant than a decameter (101). The "milli-" prefix denotes 10-3 while the "centi-" prefix denotes 10-2.

In contrast, while it is also be possible to pick more appropriate measurements of length using the US customary system of units, it is also more tedious to convert between these units, since each unit is not related by a constant power, as they are in SI. For example, the inch, foot, and yard are common measurements of length in the US customary sytem. There are 12 inches in 1 foot, and 3 feet in 1 yard. There are therefore 36 inches in 1 yard. While it is not particularly difficult to convert between units if the conversion factors are known, it is far easier in SI since it is only necessary to know what power of 10 a given prefix represents. This is true for all SI units, not just length, whereas in other systems of measurement, each different measurement can have many different units that may not be directly related by some power.