Metric lengths are all powers of ten of the meter, so converting between millimeters, centimeters and kilometers is a matter of moving a decimal point. Imperial is less tidy: 12 inches in a foot, 3 feet in a yard and 1,760 yards in a mile.
The one figure that links the two systems is the inch, which is exactly 2.54 centimeters. Everything else follows from it – a foot is 30.48cm, a yard is 0.9144m and a mile is 1,609.344m.
The UK sits awkwardly in the middle – road signs are in miles, so tape measures there often carry both scales.
Meter
The meter (m) is the SI unit of length. Since 1983 it has been defined as the distance light travels in a vacuum in 1/299,792,458 of a second.
A meter is 100 centimeters or 1,000 millimeters. It was first defined in 1791 as one ten-millionth of the distance from the North Pole to the equator, which turned out to be slightly off – that distance is nearer 10,002 kilometers.
We have many converters for meters to imperial / English units such as Meters to Yards and Meters to Feet
Centimeter
A centimeter is one hundredth of a meter, or 10 millimeters, and is written cm. An inch is exactly 2.54cm.
Divide centimeters by 100 to get meters – 175cm is 1.75m. Heights and clothing sizes are mostly given in centimeters outside the US and UK.
We have many centimeter conversion calculators on this site such as Centimeters to Inches.
Millimeters
A millimeter is one thousandth of a meter. A sheet of ordinary office paper is about 0.1mm thick, so ten sheets make a millimeter.
Converting between millimeters and other metric units is straightforward, as it is based on powers of ten. For example, there are 10 millimeters in a centimeter and 1000 millimeters in a meter. This makes millimeters a convenient unit for measuring objects that are too small to be accurately measured in centimeters or meters. If you wish to convert this unit to inches, please use our Millimeters to Inches converter.
Kilometers
A kilometer is 1,000 meters, or about 0.621 miles. Road distances in most countries are signed in kilometers, and the Earth is about 40,000 kilometers around.
Road races are usually run over 5 or 10 kilometers, and a marathon is 42.195 kilometers (about 26.2 miles).
The most common conversion we have for Kilometers is to Miles which can be found here: km to miles.
Miles
A mile is exactly 1,609.344 meters, or 1.609344 kilometers, and is divided into 1,760 yards or 5,280 feet. The word comes from the Roman mille passus, a thousand paces.
To convert miles to kilometers, multiply by 1.60934. Multiplying by 1.6 is close enough for a rough figure – 60 miles is about 96km (96.56km exactly).
Yards
The yard is 3 feet or 36 inches, and has been exactly 0.9144 meters since the international agreement of 1959. It is still used in the United States and the United Kingdom.
Fabric is still sold by the yard in the United States, and an American football field is 100 yards between the goal lines.
Feet
The foot (ft) is 12 inches, or exactly 0.3048 meters. The United States uses it for most everyday lengths and the UK for people's heights.
For example, a person's height might be given in feet and inches, a room's dimensions might be measured in square feet, or a road sign might indicate the distance to a destination in feet.
To convert feet to meters, multiply by 0.3048, or divide by 3.281 to go from meters to feet. As a rough guide a foot is 30cm and a meter is a little over 3 feet 3 inches.
Inches
An inch is exactly 2.54 centimeters, or 25.4 millimeters. It is divided by halving rather than in tens – half-inch, quarter-inch, eighth-inch and on down to sixteenths on a tape measure.
Screen sizes are quoted in inches almost everywhere, measured corner to corner – a 55-inch TV is 139.7cm.
When converting to the metric system, inches are most commonly converted into centimeters (inches to cm converter).
Another common conversion is to convert inches to millimeters.