What are length units?

Lengths are measured in two systems. The metric system, used in most of the world and in science, is built on the meter, and its other units are powers of ten of it: a millimeter is a thousandth of a meter, a centimeter a hundredth, and a kilometer a thousand meters. The imperial and US customary systems, used mainly in the United States, have inches, feet, yards, and miles: 12 inches make a foot, 3 feet a yard, and 5,280 feet a mile.

The two systems have been tied together exactly since 1959, when the international inch was set at 2.54 centimeters. That makes a foot 30.48 cm, a yard 0.9144 m, and a mile 1.609344 km.

Tool description

This tool converts lengths between millimeters, centimeters, meters, kilometers, inches, feet, US survey feet, yards, and miles. Every length converter here works the same way, from the general Length Converter to the ones for a single pair, such as millimeters to centimeters or inches to centimeters: it starts on the units in its name, and the Source and Target menus switch to any of the nine units. Type a number and the result appears as you type, ready to copy. The swap button turns the conversion around.

Units

Unit Symbol In meters
Millimeter mm 0.001
Centimeter cm 0.01
Meter m 1
Kilometer km 1,000
Inch in 0.0254
Foot ft 0.3048
US survey foot ft-us 1200/3937, about 0.304801
Yard yd 0.9144
Mile mi 1,609.344

Examples

Length Converted
1 in 2.54 cm
12 in 30.48 cm
10 mm 1 cm
100 cm 39.3701 in
1 m 3.28084 ft
6 ft 1.8288 m
1 mi 1.60934 km
1 km 0.621371 mi

How it works

The conversion is done by convert-units, a JavaScript library for unit conversion. Within one system, such as millimeters to kilometers or feet to miles, results keep 15 significant digits, which drops only floating point noise. Between the systems, convert-units uses 3.28084 feet per meter, which is exact to 6 significant digits, so results that go from metric to imperial units or back are rounded to 6 significant digits. That turns 1 inch into exactly 2.54 cm, and shows 1 mile as 1.60934 km, where the exact value is 1.609344 km.