Length Converter

How do you convert between length units?

Every length conversion comes down to one idea: turn the value into meters, then turn meters into the unit you want. Because each unit has an exact value in meters, you only need one multiplication and one division.

The two-step method

1. Multiply your value by the size of the starting unit in meters.
2. Divide the result by the size of the target unit in meters.

For example, to convert 5 feet to centimeters: 5 × 0.3048 = 1.524 m, then 1.524 ÷ 0.01 = 152.4 cm. For everyday pairs there is usually a single shortcut factor. Divide centimeters by 2.54 to get inches, multiply feet by 0.3048 to get meters with the feet to meters converter, or multiply kilometers by 0.621371 to get miles with the km to miles converter.

Metric prefixes make metric conversions easy

Metric length units differ only by powers of ten, so converting between them means moving the decimal point. Milli is one thousandth (1 mm = 0.001 m), centi is one hundredth (1 cm = 0.01 m) and kilo is one thousand (1 km = 1,000 m). Going from meters to centimeters moves the decimal two places right, and going from meters to kilometers moves it three places left.

What is the SI unit of length, and how is the meter defined?

The meter (m) is the SI base unit of length: the distance light travels in a vacuum in exactly 1/299,792,458 of a second. The BIPM SI Brochure (9th edition, 2019) states this by fixing the speed of light, c, at exactly 299,792,458 meters per second, a definition in use since 1983. Because the second is itself fixed by the cesium-133 atom, any well-equipped laboratory can realize the meter without comparing anything to a physical bar. For a sense of scale, light covers about 29.98 cm, or 11.8 inches, in one nanosecond.

How the inch and foot are tied to the meter

Customary units have no independent definition anymore. The 1959 International Yard and Pound Agreement between the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa fixed the international yard at exactly 0.9144 m. That makes the foot exactly 0.3048 m and the inch exactly 25.4 mm, the factors NIST SP 811 lists as exact. The older US survey foot, 1200/3937 m, is about 2 parts per million longer, a gap of roughly 3.22 mm over a mile. NIST and the National Geodetic Survey retired it at the end of 2022, so US mapping now uses the international foot as well.

From length to area and volume

Because area is length squared and volume is length cubed, these exact factors carry straight over. A square foot is exactly 0.09290304 m² (0.3048 × 0.3048), and a cubic foot is exactly 0.028316846592 m³. The area converter and the volume converter are built on the same definitions, so a room measured in feet converts cleanly to square meters or cubic meters.

How do inches, feet, yards and miles relate to each other?

There are 12 inches in a foot, 3 feet in a yard, and 1,760 yards, or 5,280 feet, in a mile, and every one of those steps is exact. The customary ladder is irregular, which is why it trips people up, while the metric ladder only ever moves the decimal point. The table gives each relationship with its exact metric value.

For everyday jobs, use the feet to inches converter for the 12-to-1 step, the inches to cm converter for small parts and screen sizes, and the feet to meters converter for floor plans and building work.

A mental shortcut for kilometers and miles

Consecutive Fibonacci numbers sit close to the km-to-mile ratio: 5 km ≈ 3.11 mi (call it 3), 8 km ≈ 4.97 mi (call it 5), and 13 km ≈ 8.08 mi (call it 8). Multiplying by 0.6 is the other quick estimate; it undershoots by about 3.4%, so 10 km comes out as 6 miles instead of 6.21. For exact race and road distances, the km to miles converter does the division, and the speed converter applies the same factor to mph and km/h.

Where is each length unit used?

Almost every country measures length in meters and its metric multiples, while the United States still uses inches, feet, yards and miles for everyday lengths, and the UK keeps miles for roads. Many industries also keep a traditional unit worldwide, so the right unit depends on the job as much as the country.

• Millimeters and centimeters: product dimensions, clothing, paper sizes and medical measurements in most countries.
• Inches: screen and TV sizes, tire and wheel sizes, and plumbing fittings worldwide, plus most everyday lengths in the US.
• Feet and inches: human height, room dimensions and aircraft altitude in the US, UK and aviation.
• Meters and kilometers: athletics, construction and road distances almost everywhere outside the US.
• Miles: road distances and speed limits in the US and UK.
• Nautical miles: sea and air navigation, because one nautical mile matches one minute of latitude.
• Micrometers and nanometers: cell biology, semiconductor chips and wavelengths of light.