About Length Converter
Convert between mm, cm, m, km, inches, feet, yards, and miles instantly. Free, no sign-up required.
How to use
- Type your length into the Value field. Decimals and very small or very large numbers are both fine — the converter switches to scientific notation automatically when results drop below 0.01 of the chosen unit.
- Pick the source unit from the From dropdown. Available: millimeters (mm), centimeters (cm), meters (m), kilometers (km), inches (in), feet (ft), yards (yd), and miles (mi).
- Choose the target unit in the To dropdown. The most common pairings are inches-to-cm for hardware, feet-to-meters for construction, and miles-to-kilometers for travel — but any combination works.
- Use the swap button between the two dropdowns to instantly reverse direction. This is fastest for round-tripping a value to confirm you used the right factor.
- Read both the converted value (in the highlighted box) and the formula line below it, which restates the conversion explicitly — useful when documenting a calculation in a spec sheet or homework problem.
- Remember the international foot is exactly 0.3048 m by treaty since 1959, and the international mile is exactly 1,609.344 m. All length conversions in this tool are exact-by-definition; only the displayed digits are rounded.
Examples
Travel distance
A 100 mile road trip equals 160.934 km. The factor 1.609344 comes straight from the 1959 international yard and pound agreement. For quick mental math, multiply miles by 1.6 — accurate enough for almost any everyday estimate.
Rebar specification
A 5/8 inch rebar (0.625 in) is 15.875 mm — close to but not equal to a metric M16 spec at 16 mm. North American construction often calls out imperial rebar sizes that translate to non-round metric values, which is why dual-spec drawings list both.
Astronomical scale
A nautical mile (1,852 m) is exactly one minute of latitude. That is why marine and aeronautical charts use them — distance directly equals angular position. 60 nautical miles equals 1 degree of latitude, regardless of where on Earth you are.
Frequently asked questions
How do I convert inches to centimeters?
Multiply inches by 2.54 to get centimeters. So 12 in = 30.48 cm, and 1 in = 25.4 mm exactly. The factor 2.54 is exact by international agreement (1959 international inch). Going the other way, divide cm by 2.54. This shows up constantly in plumbing fittings, lumber thicknesses, paper sizes, and any spec written in mixed units.
How many feet are in a meter?
1 m = 3.280840 ft, or roughly 3 ft 3⅜ in. Conversely, 1 ft = 0.3048 m exactly. The meter has been defined by the speed of light since 1983: it is the distance light travels in vacuum in 1/299,792,458 of a second. The foot piggybacks on that definition — it is exact, not approximate.
What is the difference between metric and imperial length units?
Metric (mm, cm, m, km) uses powers of 10: 1 m = 100 cm = 1,000 mm, and 1 km = 1,000 m. Imperial uses irregular ratios: 12 in = 1 ft, 3 ft = 1 yd, 1,760 yd = 1 mi (or 5,280 ft = 1 mi). Most countries use metric for science, engineering, and trade. The US, Liberia, and Myanmar still use imperial as the everyday system, though the US scientific and military communities use metric internally.
What is a nautical mile and why is it different?
1 nautical mile = 1,852 m exactly = 1.15078 statute miles. It corresponds to one minute (1/60 of a degree) of latitude on Earth's surface. Aviation and maritime navigation use nautical miles because angular position on a chart maps directly to distance — no conversion needed. Aircraft speed in knots (nautical miles per hour) inherits the same advantage. This converter does not include nautical miles by default; use the Speed Converter for knots.
How big is a light-year compared to a kilometer?
1 light-year = 9.461 × 10¹² km = 9.461 × 10¹⁵ m. It is the distance light travels in vacuum in one Julian year (365.25 days) at exactly 299,792,458 m/s. Within our solar system, astronomers use astronomical units (AU = 149.6 million km); for stars they use light-years or parsecs (1 pc ≈ 3.26 ly). At terrestrial scales kilometers are still king.
Why do US construction drawings sometimes use 0.083 ft instead of 1 inch?
Some US engineering and surveying disciplines use decimal feet rather than feet-and-inches because it makes calculations easier on calculators and CAD systems. 0.083 ft = 1 in (since 1/12 ≈ 0.0833). You will see it in civil engineering plans, road surveys, and grading specs. Carpentry and framing stick with feet-and-inches because it lines up with tape measure markings.
Are length conversions exact?
Yes — every conversion factor in this tool is exact by international definition. The international yard (0.9144 m), foot (0.3048 m), and inch (0.0254 m) were fixed by the 1959 international agreement. The mile (1,609.344 m) and nautical mile (1,852 m) are also exact. Rounding only appears in the displayed digits; the underlying double-precision arithmetic is good to about 15 significant figures.
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