Length Unit Converter

Convert between any pair of length units — meter, kilometer, centimeter, millimeter, inch, foot, yard, mile, and more — using factors fixed by the 1959 international yard/pound agreement and verified against NIST SP 811. Type a value and the result updates instantly.

Reviewed by WuTools Engineering Team · Last updated 2026-05-07

Popular conversions

Why are there so many length units, and how are they related?

Length is the most fundamental physical measurement humans make: it answers "how far", "how tall", "how long". The SI base unit for length is the meter (m), defined since 1983 as the distance light travels in vacuum during 1/299,792,458 of a second — a definition tied to the speed of light, the most stable constant we know. Every other length unit is now defined as an exact multiple or fraction of the meter.

Two parallel families dominate everyday length: the metric system (millimeter, centimeter, meter, kilometer) used by almost every country, and the imperial / US-customary system (inch, foot, yard, mile) used in the United States and a few other places. Since 1959 the two systems are linked by exact integer-ratio definitions: 1 inch = 25.4 mm exactly, so every imperial conversion against metric is a precise rational number, not an approximation.

The length units, explained

Meter (m) — the SI base

The meter is the SI base unit of length. Originally one ten-millionth of the distance from the equator to the North Pole, it was redefined in 1983 in terms of the speed of light. Most engineering, science and construction work in metric countries uses the meter directly or one of its decimal multiples (mm, cm, km).

Centimeter (cm) and millimeter (mm) — the workhorses

1 cm = 10 mm = 0.01 m. The centimeter dominates everyday measurement (height of a person, paper sizes), the millimeter is the standard for engineering drawings and machining tolerances. Almost every country except the US specifies clothing, paper, screws, and consumer products in cm or mm.

Kilometer (km) — distance and travel

1 km = 1,000 m. Used for road distances, running and cycling events, geography, and astronomy at small scales. 1 km ≈ 0.621 mi, so a 5K run is about 3.1 miles.

Inch (in) — the fundamental imperial unit

Defined since 1959 as exactly 25.4 mm. Used in the US for screen diagonals (a "15-inch laptop"), tire diameters, plumbing, screws, and many engineering specs worldwide. The inch underlies all other imperial length units: 12 in = 1 ft, 36 in = 1 yd, 63,360 in = 1 mi.

Foot (ft) and yard (yd) — building and sport

1 ft = 12 in = 0.3048 m exactly; 1 yd = 3 ft = 0.9144 m. The foot is the standard for human height in the US and UK (5 ft 10 in is a common adult height). Yards are still the unit of choice in American football fields (100 yd between goal lines) and many fabric measurements.

Mile (mi) and nautical mile (nmi) — long distance

The statute mile is 5,280 ft = 1,609.344 m, used for road and running distances in the US and UK. The nautical mile is exactly 1,852 m — defined as 1 minute of arc along a meridian — and is the unit for marine and air navigation worldwide. Speed in knots = nautical miles per hour.

Micrometer (μm) and nanometer (nm) — small scale

1 μm = 0.001 mm = 1,000 nm. Used for cell biology (a red blood cell is ~7 μm), semiconductor process nodes (now in the nm range), and light wavelength (visible light spans ~400–700 nm).

Real-world applications and which unit to expect

Human height: Metric countries quote height in cm (170 cm) or m (1.70 m). The US and UK use feet and inches (5 ft 7 in). 5 ft 7 in ≈ 170.18 cm.
Road distances: Metric countries: km on signs and odometers. United States and UK: mi. 100 km/h ≈ 62 mph; the speed-limit "60" sign is two completely different speeds depending on country.
Screens and TVs: Display diagonals are universally inches even in metric countries. A 24-inch monitor is 60.96 cm diagonally.
Construction and DIY: Lumber sizes in the US are nominal inches (a "2×4" is actually 1.5 × 3.5 in / 38 × 89 mm). Metric construction uses mm exclusively for plans and m for room dimensions.
Marine and aviation navigation: All countries use nautical miles for ocean and air distances; speeds in knots. Visibility is reported in km in metric countries, statute miles in the US.
Manufacturing tolerances: Engineering drawings specify in mm with tolerances often ±0.01 mm or tighter. Imperial drawings use inches with tolerances in thousandths ("thou") — 0.001 in = 0.0254 mm.
Sports: Olympic track is 400 m; American football is 100 yd; tennis court is 23.77 m / 78 ft; pool length is 50 m or 25 yd.

How much is 1 unit of each in meters?

Unit	Value in meters (m)
1 nm (Nanometer)	0.000000001 Pa
1 μm (Micrometer)	0.000001 Pa
1 mm (Millimeter)	0.001 Pa
1 cm (Centimeter)	0.01 Pa
1 m (Meter)	1 Pa
1 km (Kilometer)	1000 Pa
1 in (Inch)	0.0254 Pa
1 ft (Foot)	0.3048 Pa
1 yd (Yard)	0.9144 Pa
1 mi (Mile)	1609.344 Pa
1 nmi (Nautical mile)	1852 Pa

Frequently asked questions about length units

How do I convert between any two length units?

Multiply by the source unit's meter-factor and divide by the target's. Example: convert 12 inches to centimeters → 12 × 0.0254 ÷ 0.01 = 30.48 cm. The converter above does this in real time.

Is 1 inch exactly 2.54 cm?

Yes — by international agreement since 1959, 1 inch = 25.4 mm = 2.54 cm exactly. This is a definition, not a measurement, so all imperial-to-metric length conversions are exact rational numbers.

How many feet are in a meter?

1 m = 1 ÷ 0.3048 ≈ 3.2808399 ft. So 1 meter is just over 3 feet 3 inches. A common shortcut: meters × 3.28 ≈ feet, accurate to within 0.03%.

How many kilometers are in a mile?

1 mile = 1.609344 km exactly, and 1 km = 0.621371192 mi. A 5K race is 3.107 miles; a marathon (42.195 km) is 26.219 miles.

What is the difference between a statute mile and a nautical mile?

A statute mile (1.609344 km) is the legal mile used on land in the US and UK. A nautical mile (1.852 km) is exactly one minute of arc along a meridian and is the universal unit at sea and in the air. 1 nmi ≈ 1.151 statute miles.

How do I read a height like 5'7"?

It means 5 feet 7 inches. Convert by multiplying feet by 12, adding the inches, then multiplying by 2.54 cm: (5 × 12 + 7) × 2.54 = 170.18 cm or 1.70 m.

Why does the US still use inches and feet?

Switching costs: every road sign, building plan, screw size, and product label would need to change. Several attempts at metric adoption stalled in the 1970s. Some technical fields in the US (medicine, science, the military, motorsport) already use metric internally.

Are there length units I should never see?

The chain (66 ft), furlong (660 ft) and rod (16.5 ft) appear only in old land surveys. The fathom (6 ft) is occasionally used in marine work. None of these are needed in modern engineering — stick to mm, m, km, in, ft, yd, mi.

How precise are these conversion factors?

All factors used by this converter are exact by definition: 1 in = 25.4 mm, 1 ft = 0.3048 m, 1 mi = 1609.344 m, 1 nmi = 1852 m. The displayed result is rounded to ten significant digits, far beyond what any real measurement requires.