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Length converter

Convert length units instantly — cm, m, km, inches, feet, yards and miles, all at once with exact factors.

Sources: NIST — Guide to the SI, Appendix B.8 (conversion factors: inch, foot, yard, mile)

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Popular conversions

Millimeters (mm)
Centimeters (cm)
Meters (m)
Kilometers (km)
Inches (in)
Feet (ft)
Yards (yd)
Miles (mi)
Nautical miles (nmi)

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What the length converter does

The length converter turns a value in one unit into all the others at once — millimeters, centimeters, meters, kilometers, inches, feet, yards, miles and nautical miles. Type a number, pick the unit you’re converting from and the unit you’re converting to, and every other unit updates below it immediately. No separate tool for “cm to feet” and another for “km to miles” — one converter, nine units, always linked.

It’s built for the moments a unit mismatch actually costs you something: a height quoted in centimeters that you need in feet and inches, a part measured in inches on a blueprint calibrated in millimeters, a race distance in kilometers you want to picture in miles. The calculator uses exact definitional conversion factors, not rounded approximations, so the numbers hold up even at high precision — engineering drawings, GPS work, anything where “close enough” isn’t.

How to use the length converter

  1. Enter a value. Any number, zero or greater, in any of the nine supported units.
  2. Pick the “From” unit. The unit your value is currently in.
  3. Pick the “To” unit. The unit you want as the headline result — the calculator still shows every unit at once, but this one is highlighted.
  4. Read the result. The converted value appears instantly, next to the same input expressed in all eight other units, so you can compare without retyping anything.

There’s no “convert” button and no page reload — the result updates as you type.

How the conversion works

Every unit converts through meters, the calculator’s canonical unit. To go from unit A to unit B, the calculator multiplies the input by A’s meter-factor, then divides by B’s meter-factor — one multiplication, one division, nothing rounds in between.

The meter-factors themselves come from exact legal definitions, not from measurement:

  • 1 inch = 0.0254 meters, exactly. This is set by the International Yard and Pound Agreement of 1959 (codified in NIST SP 811, Appendix B.8), which fixed the inch, foot, yard and mile as exact multiples of the meter for the US, UK and four other countries.
  • 1 foot = 12 inches = 0.3048 m, 1 yard = 3 feet = 0.9144 m, and 1 mile = 1,760 yards = 1,609.344 m — all exact, since each is defined as a whole-number multiple of the inch.
  • 1 nautical mile = 1,852 meters, exactly, fixed by the 1929 International Extraordinary Hydrographic Conference and used worldwide in aviation and marine navigation.
  • Metric units (mm, cm, m, km) are exact decimal multiples of the meter by definition — that’s the whole point of the metric system.

Worked example

Convert 100 cm to feet:

  1. 100 cm × 0.01 (the cm-to-meter factor) = 1 m.
  2. 1 m ÷ 0.3048 (the meter-to-foot factor) = 3.28084 ft.

That same 1 m also equals 39.3701 in, 1.09361 yd and 0.000621371 mi — the length converter shows all of them from that single input, without any extra steps.

Metric vs. imperial

Metric units scale by powers of ten — 1 m = 100 cm = 1,000 mm — so moving between them is just shifting a decimal point. Imperial units scale by the traditional factors above (12, 3, 1,760), inherited from historical measurement systems and now frozen in place by the 1959 agreement. That mismatch of bases is exactly why a general-purpose converter earns its keep: nobody keeps “a mile is 5,280 feet and also 1,609.344 meters” in their head.

Where length conversion comes up

Height is the most common one — medical forms, ID documents and casual conversation split between centimeters and feet-and-inches depending on the country. Construction and interior design mix meters and feet depending on which plans or tools are in front of you. Manufacturing and 3D printing move constantly between millimeters and inches, because tooling standards differ by region. Race and route distances are published in kilometers in most of the world and miles in the US and UK — if you’re converting a running pace rather than a one-off distance, the running pace calculator handles that directly. Nautical miles show up specifically in aviation and marine navigation, where they line up with minutes of latitude.

Frequently asked questions

Why isn’t 1 inch just “about” 2.54 cm?

It’s exactly 2.54 cm — 0.0254 meters — not an approximation. The International Yard and Pound Agreement of 1959 fixed this value by international treaty, so the US, UK, Canada, Australia and other countries all use the identical exact figure. Any tool that shows a slightly different conversion is rounding somewhere it shouldn’t.

What’s the difference between a mile and a nautical mile?

A statute mile is exactly 1,609.344 meters (1,760 yards) — the standard “mile” used on land in the US and UK. A nautical mile is exactly 1,852 meters, about 15% longer, and is used in aviation and sailing because it corresponds to one minute of latitude on the Earth’s surface. They’re not interchangeable, and mixing them up on a flight plan or nautical chart matters.

How many decimal places should I trust in the result?

The calculator computes with full floating-point precision and displays up to six significant digits — far more than a tape measure or a road sign gives you. For everyday use (height, furniture, driving distances) two or three significant digits are plenty; the extra precision mainly matters for engineering, machining or scientific work.

Does converting a value and then converting it back give the same number?

Yes. Converting 170 cm to feet and then that result back to centimeters returns 170 cm, because both directions use the same exact factor, just inverted. What can look different is the displayed rounding — six significant digits of a long decimal can format differently depending on which unit is on top.

Can I convert zero or negative values?

Zero converts to zero in every unit — that’s a valid input, not an error. Negative lengths aren’t physically meaningful, so the calculator expects a value of zero or more.

Which unit should I start from for very small or very large lengths?

It doesn’t matter — enter the value in whatever unit you already have, and every other unit is available in the result at once. Millimeters suit precise, small-scale work like parts, print or jewelry; kilometers and miles suit travel and route planning.

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