How length conversion works
To convert between any pair of length units, the simplest approach is to use the meter as an intermediate step, since it is the base unit of the International System. The input value is first turned into meters, and that total is then converted into the desired target unit. That is exactly how this calculator works behind the scenes.
Conversion factors used
1 meter = 1,000 millimeters = 100 centimeters
1 kilometer = 1,000 meters
1 inch = 2.54 centimeters (exact definition)
1 foot = 12 inches = 0.3048 meters
1 yard = 3 feet = 0.9144 meters
1 mile = 1,760 yards = 1,609.344 meters
Worked example
A 10 kilometer run equals roughly 6.21 miles, since 10 multiplied by 0.621371 gives that value. In meters, the same 10 kilometers correspond to 10,000 meters, because 1 kilometer is exactly 1,000 meters.
Metric and imperial systems
The metric system, used across most of the world, is based on powers of 10: millimeters, centimeters, meters and kilometers follow multiples of 10, 100 and 1,000. The imperial system, used mainly in the United States, has less intuitive relationships between its units, such as 12 inches in a foot and 3 feet in a yard. Because of this, conversions between the two systems often produce numbers with many decimal places, unlike conversions within the metric system itself.
Converting a height in feet and inches
In the United States and other countries that use the imperial system, it is common to express a person's height in feet and inches, like "5 feet 9 inches". To convert that measurement to centimeters, everything is first turned into inches, by multiplying the feet by 12 and adding the inches, and then that total is multiplied by 2.54. The calculator on this page runs that calculation automatically, showing the result in centimeters, meters and inches.