Length Converter

The converter behaves like a calculator that already knows every factor, and it updates as you type.

1. Type a number in the Value box. Decimals work, and so do negative numbers, which suit differences or offsets.
2. Choose the unit you have in From and the unit you want in To. The answer appears in the large result area straight away.
3. Read the line under the result, which gives the one-unit factor in both directions, such as 1 mi = 1.6093 km.
4. Press Swap units to reverse the direction without retyping the number.
5. Look at the In every unit list on the right for the same length in all nine units, which helps when a plan or spec sheet asks for a different one.

Results are shown to four decimal places between 1 and 1,000, to two decimals above 1,000 and to six below 1, so a long exact value such as 8.04672 appears as 8.0467. Everything runs in your browser and nothing you enter is sent anywhere.

The imperial and US customary units are defined in terms of the metre, under the international yard and pound agreement of 1959. The converter stores each unit as its length in metres and works through that base, so no rounding creeps in between steps.

The foot used is the international foot. The older US survey foot differs from it by about two parts per million, which matters only on very large land surveys. The nautical mile began as one minute of latitude and is now fixed at 1,852 metres, which is why it is the unit of distance at sea and in aviation.

These examples can be reproduced in the converter. Remember that the display rounds, so 5 miles shows as 8.0467 km although the exact value is 8.04672 km.

• 10 km is 6.2137 miles, and a marathon of 42.195 km is 26.2188 miles.
• A height of 5 ft 10 in has no single box, so enter 70 inches and convert to centimetres: 177.8 cm. In the other direction, 180 cm is 5.9055 ft, and the 0.9055 ft left over is 10.87 in, so 5 ft 10.87 in.
• A 6.1 inch phone screen is 154.94 mm along its diagonal.
• A4 paper at 210 by 297 mm is 8.2677 by 11.6929 inches.
• 1 nautical mile is 1.852 km and 1.1508 statute miles.

The most common mistake is treating decimal feet as feet and inches. A height of 5.10 ft means 5.1 feet, which is 155.4 cm, not 5 ft 10 in. Another is mixing up the statute mile used on roads with the nautical mile used at sea, which are about 15 percent apart. A third is trusting a rounded memory aid such as 1 inch = 2.5 cm on a long run of measurements, where the small error adds up: over 100 inches it is already 4 cm out. A fourth is entering a length in the wrong box, so glance at the unit names in From and To before copying a result.

This page converts a single length. If your quantity is an area, such as a plot or a room, use the area converter instead, because squaring a length factor by hand is a common source of error. For distance covered per unit of time, the speed converter handles miles per hour and kilometres per hour, and the running pace calculator turns a distance and a time into a pace per kilometre or mile. Body height and weight go into the BMI calculator, which accepts both metric and imperial input, so converting first is not necessary. For multi-step arithmetic on lengths, the scientific calculator lets you write a calculation such as 5 * 1.609344 on one line. Values are accepted between minus and plus one trillion.