Length Converter
Convert between metric and imperial lengths: mm, cm, m, km, inches, feet, yards and miles.
1 cm = 0.3937008 in
Quick conversions
This converts between metric and imperial length units: millimetres, centimetres, metres, kilometres, inches, feet, yards and miles.
The imperial-metric relationships are exact by definition, not approximations.
How length units relate
Length conversion is a single multiplication, and the useful thing to know is that the key conversions are exact by definition rather than rounded measurements.
Since the 1959 international yard and pound agreement, one inch is defined as exactly 2.54 cm. From that, one foot is exactly 0.3048 m and one mile is exactly 1.609344 km. These are not approximations that could be refined with better instruments; they are definitions.
The metric side is easier because it is all powers of ten: 10 mm to a centimetre, 100 cm to a metre, 1,000 m to a kilometre. Imperial is 12 inches to a foot, 3 feet to a yard, and 1,760 yards to a mile, which is why converting within imperial is often harder than converting between systems.
Set by international agreement in 1959. Everything else in the imperial system follows from the inch.
What to enter
- Value and unit
- The measurement to convert, and what unit it is in.
- Target unit
- What to convert to. The tool handles feet-and-inches as a combined input.
Conversion factors
- 1 inch
- 2.54 cm, or 25.4 mm. Exact.
- 1 foot
- 0.3048 m, or 30.48 cm. Exact. Twelve inches.
- 1 yard
- 0.9144 m. Exact. Three feet.
- 1 mile
- 1.609344 km. Exact. 5,280 feet or 1,760 yards.
- 1 kilometre
- 0.621371 miles. Useful for reading race distances.
What this assumes
These are international (US customary) units. The UK nautical mile and the US survey foot are separate definitions that differ very slightly.
A nautical mile is 1,852 m exactly, which is a different unit from the statute mile and is used in aviation and at sea.
How to calculate length conversion
Multiply by the conversion factor, or divide to go the other way.
- reverse
- Divide instead of multiplying, or use the reciprocal
- feet and inches
- Convert to total inches first: 5 ft 10 in = 70 in
Convert compound measurements to one unit first. 5 ft 10 in becomes (5 × 12) + 10 = 70 inches. Trying to convert feet and inches separately is where errors start.
Multiply by the factor. 70 × 2.54 = 177.8 cm.
Divide to reverse. 177.8 ÷ 2.54 = 70 inches, which converts back to 5 ft 10 in.
Use the mental shortcuts for estimates. A kilometre is about 0.6 miles, a metre is a bit over a yard, and a centimetre is about 0.4 inches.
See a worked example: height and distance
- Height
- 5 ft 10 in
- Distance
- 5 km
Height: (5 × 12) + 10 = 70 inches, × 2.54 = 177.8 cm.
For comparison, 6 ft is 72 inches, which is 182.9 cm.
Distance: 5 × 0.621371 = 3.107 miles, which is why a 5K is just over three miles.
A marathon at 42.195 km is 26.219 miles by the same conversion.
177.8 cm and 3.107 miles
Frequently asked questions
Exactly 2.54. It is a definition, agreed internationally in 1959, not a measured approximation.
Everything else in the imperial system follows: a foot is 30.48 cm, a yard is 91.44 cm, and a mile is 160,934.4 cm.
Convert to total inches first, then multiply by 2.54. 5 ft 10 in is 70 inches, so 177.8 cm.
Going back, divide by 2.54 for total inches, then divide by 12 for feet with the remainder as inches.
3.107 miles. A kilometre is 0.621371 miles, so 5 × 0.621371 = 3.107.
The other common race distances: 10K is 6.214 miles, a half marathon is 13.109, and a marathon is 26.219.
A kilometre is about 0.6 miles, and miles to kilometres is roughly × 1.6. A metre is a bit over a yard, and 4 inches is about 10 cm.
For a better mile-to-kilometre estimate, use the Fibonacci trick: consecutive Fibonacci numbers approximate the ratio, so 5 miles is about 8 km and 8 miles is about 13 km.
1,852 metres exactly, about 1.15 statute miles. It is used in aviation and at sea.
It was originally defined as one minute of latitude, which makes navigation arithmetic on a chart much simpler. Knots are nautical miles per hour.
Historical inertia. The 1975 Metric Conversion Act made metrication voluntary rather than mandatory, and it never completed.
US customary units are legally *defined* in metric terms, so the country is metric underneath. Science, medicine and the military all use metric routinely.
Problems people actually run into
Converting feet and inches separately
5 ft 10 in is not 5 × 30.48 plus 10 × 2.54 done casually and recombined wrongly. Convert to 70 total inches first.
Compound imperial measurements need flattening to a single unit before any conversion. It removes the whole class of error.
Treating 2.54 as an approximation
It is exact by definition, so rounding it to 2.5 introduces a real 1.6% error rather than a negligible one.
On a 100-inch measurement that is more than four centimetres. Use the full factor for anything that has to fit.
Results are estimates for general information only and are not professional financial, medical, or legal advice. Read our full disclaimer.
Last updated: September 4, 2026