A unit of measurement is a standardized quantity of a physical property. For example, a foot is a unit of measurement because it precisely defines the length of an object. An inch is another unit of measurement that also precisely defines the length of an object. But one foot and one inch are different lengths, and it can be useful to convert between the two to compare two lengths. This is called unit conversion.

To do a unit conversion, get how many of one unit can fit into the other unit. For feet and inches, there are 12 inches in 1 foot, so to convert 2 feet to inches, do the following:

feet   = 2
inches = 2 × 12 = 24

There are two main systems of measurement: metric and imperial. Metric is used by most of the world, and its units of measurement differ by a factor of 10, making it easy to do unit conversions. For example, the metric unit of length is a meter. There are 100 centimeters in 1 meter, 1,000 millimeters in 1 meter, and 1,000 meters in 1 kilometer. Imperial is used by three countries only: Liberia, Myanmar, and the USA. Its units of measurements are based on the human body, making it hard to do unit conversions since the conversions are haphazard. For example, there are 5,280 feet in 1 mile.

Unit conversion is used to both convert units within a system of measurement, for example, feet to inches, and to convert a unit in one system to a unit in another system, for example, inches to centimeters. Converting from one system to another is usually messier. For example, to convert 2 inches to centimeters, do the following:

inches      = 2
centimeters = 2 × 2.54 = 5.08