How length conversion works
Every length unit on this page is defined as an exact number of meters. The meter is the SI base unit of length, and since 1959 the inch, foot, yard and mile used in the United States, the United Kingdom and the rest of the English-speaking world have been fixed to it: the international yard is exactly 0.9144 m, which makes the foot 0.3048 m and the inch 2.54 cm. Nothing about these factors is rounded or measured; they are definitions.
That means converting between any two units takes two steps, which the converter does for you:
- Turn the value into meters by multiplying by the meter value of the starting unit.
- Divide by the meter value of the target unit.
For a direct pair such as inches to centimeters, the two steps collapse into one factor (2.54), which is what the working line under the result shows.
Formulas
All four factors are exact. The reverse factors (1 cm = 0.393700787… in, 1 km = 0.621371192… mi) never end, which is why dividing by the exact factor gives cleaner results than multiplying by a rounded reciprocal.
The units in the converter, with their exact size in meters:
| Unit | Symbol | Meters |
|---|---|---|
| Millimeter | mm | 0.001 |
| Centimeter | cm | 0.01 |
| Inch | in | 0.0254 |
| Foot (12 in) | ft | 0.3048 |
| Yard (3 ft) | yd | 0.9144 |
| Kilometer | km | 1,000 |
| Mile (5,280 ft) | mi | 1,609.344 |
| Nautical mile | nmi | 1,852 |
Worked examples
- Put everything in inches first: 5 ft × 12 = 60 in, plus 9 in = 69 in.
- Multiply by 2.54: 69 × 2.54 = 175.26 cm.
- A mile is 1.609344 km, so divide: 10 ÷ 1.609344 = 6.2137119…
- Round to a sensible precision for running: 6.21 miles.
- Convert to inches: 180 ÷ 2.54 = 70.866 in.
- Take out whole feet: 70.866 ÷ 12 = 5 remainder 10.866, so 5 ft and 10.866 in.
- Round the inches, not the feet: 5 ft 10.9 in, or 5 ft 11 in to the nearest inch.
Conversion tables
| Inches (in) | Centimeters (cm) |
|---|---|
| 1 | 2.54 |
| 2 | 5.08 |
| 3 | 7.62 |
| 4 | 10.16 |
| 5 | 12.7 |
| 6 | 15.24 |
| 8 | 20.32 |
| 10 | 25.4 |
| 12 | 30.48 |
| 18 | 45.72 |
| 24 | 60.96 |
| 36 | 91.44 |
| Miles (mi) | Kilometers (km) |
|---|---|
| 1 | 1.609344 |
| 2 | 3.218688 |
| 3 | 4.828032 |
| 5 | 8.04672 |
| 10 | 16.09344 |
| 13.1094 | 21.097534 |
| 26.2188 | 42.195068 |
| 50 | 80.4672 |
| 100 | 160.9344 |
Common mistakes
- Dividing feet and inches as a decimal. 5 ft 9 in is 5.75 ft, not 5.9 ft, because a foot has 12 inches, not 10. Convert to inches first, as in the first example.
- Rounding the factor. Using 2.5 cm per inch or 1.6 km per mile is fine for a rough guess, but it drifts: 1.6 turns a marathon into 41.95 km instead of 42.195 km.
- Mixing up miles and nautical miles. A nautical mile (1,852 m) is about 15% longer than a statute mile (1,609.344 m). Ships and aircraft use nautical miles; road signs use statute miles.
- Treating the old US survey foot as the same unit. Land surveys in the United States used a survey foot of 1200/3937 m until it was retired at the end of 2022. It differs from the international foot by about 2 parts per million, which matters only over long survey distances, but old property records may still be in survey feet.
The converter shows up to ten significant digits. Measurements are rarely that precise: if a tape measure reads 69 in, give the answer as 175 cm or 175.3 cm, not 175.26 cm.
Questions people ask
How many centimeters are in a foot?
Exactly 30.48 cm, because a foot is 12 inches of 2.54 cm each.
Is a meter longer than a yard?
Yes. A meter is about 1.0936 yards, so 100 m is roughly 109.4 yd. On a running track, the 100 m dash is about 9 yards longer than a 100-yard dash.
Why do some countries still use inches and feet?
The United States uses US customary units for most everyday lengths, and the United Kingdom still uses miles on roads and feet and inches for personal height. Both define these units through the meter, so conversions between them are exact.
Sources
- Guide for the Use of the International System of Units (SI), NIST Special Publication 811, Appendix B, National Institute of Standards and Technology.
- The International System of Units (SI Brochure), 9th edition, Bureau International des Poids et Mesures.
- U.S. Survey Foot, National Institute of Standards and Technology.