Length Converter

Length is the oldest measurement problem humans have tried to standardize. The metric system solved most of it in 1795; the US, Liberia, and Myanmar are the three holdouts. Everyone else works in meters and millimeters by default and only reaches for feet and inches when dealing with American hardware, TV screens, or tire sizes.

A meter was originally defined in 1799 as one ten-millionth of the distance from the North Pole to the equator through Paris. That definition was off by about 0.2 mm (the surveyors made a small arithmetic error). It was then tied to a platinum bar kept in a vault outside Paris, then to the wavelength of krypton-86 light, and since 1983 to the distance light travels in 1/299,792,458 of a second. The foot, by contrast, was whatever the local king's foot happened to be until 1959, when the US and UK agreed the international yard would be exactly 0.9144 meters.

That 1959 agreement matters more than it sounds. Pre-1959 US survey feet are still used in some state plane coordinate systems, differ from international feet by about 2 parts per million, and have caused real surveying disputes when boundary markers from different eras don't line up. If you are reading a property deed older than 1959, check whether the surveyor used the pre-1959 foot.

The conversion table below the calculator is the fastest way to sanity-check a length. Enter the number once; read it in every common unit at once. Feet-to-meters is roughly divide-by-3.28 (or multiply by 0.305). Miles-to-kilometers is multiply by 1.609, or use the Fibonacci trick: 3 mi ≈ 5 km, 5 mi ≈ 8 km, 8 mi ≈ 13 km, 13 mi ≈ 21 km. Each pair is a successive Fibonacci number, and the ratio happens to be very close to the golden ratio, which is very close to the mile-to-kilometer conversion factor.

For very small lengths, remember the SI prefix ladder steps by factors of 1000: millimeters → micrometers → nanometers → picometers. A human hair is about 70 micrometers thick. A red blood cell is about 8 micrometers. Visible light wavelengths sit around 400-700 nanometers. A silicon atom is about 0.2 nanometers across.

• Meters
• Kilometers
• Centimeters
• Millimeters
• Inches
• Feet
• Yards
• Miles

• Confusing 'foot' and 'feet' in imports. A 2400 mm kitchen cabinet is not 2400 feet. Obvious here, less obvious when copying a spec sheet under deadline pressure.
• Assuming a 'meter' on a Chinese datasheet is the SI meter. It almost always is — but historical Chinese units also used 'chi' (about 1/3 meter) and 'zhang' (about 3.33 meters). Recent datasheets are SI; antique furniture catalogs are not.
• Reading nautical miles as statute miles. One nautical mile is 1.852 km, or about 1.151 statute miles. A 100-knot wind is 115 mph, not 100 mph. This matters for boats and planes.
• Mixing 'hand' units. Horse heights are measured in hands (4 inches), and a '15.2 hand' horse is not 15.2 hands — it's 15 hands and 2 inches, so 62 inches or 1.575 m. The decimal is a base-4 remainder, not a fraction of a hand.
• Pre-1959 US survey feet. About 2 parts per million off from international feet. Irrelevant for most work, catastrophic in long-baseline surveying.

• iPhone 15 Pro is 146.6 mm tall — about 5.77 inches.
• A US letter page is 11 inches (279.4 mm) tall; A4 is 297 mm.
• NFL football field is 109.7 m (360 ft) from back of one end zone to the other.
• Boeing 747-8 is 76.3 m (250 ft) long — just over 2.5 football fields' worth of length fits nose-to-tail.
• Empire State Building is 381 m (1250 ft) to the roof, 443 m (1454 ft) to the tip.
• Marathon distance is exactly 42.195 km (26.219 mi) — set at the 1908 London Olympics so the race could finish in front of the royal box.
• Everest is 8,848.86 m (29,031.7 ft) above sea level as of the 2020 Nepal-China joint survey.

• Memorize five anchors: 1 inch = 2.54 cm exactly; 1 foot = 30.48 cm; 1 yard = 0.9144 m; 1 mile = 1.609 km; 1 nautical mile = 1.852 km.
• Room dimensions: a 10 x 12 ft bedroom is 3.05 x 3.66 m, or about 11.2 m². Good for checking rental listings across countries.
• Body-scale estimates: the span of an adult hand (pinky to thumb, fully spread) is close to 9 inches or 23 cm. The last joint of your index finger is close to 1 inch or 2.5 cm. Your outstretched arms end-to-end are roughly your height.

Related: Area Converter · Volume Converter · Angle Converter.