Convert between metres, kilometres, miles, feet and inches instantly.
This converts between metric and imperial units of length, from millimetres to miles. Every conversion routes through the metre, which is defined as the distance light travels in a vacuum in 1/299,792,458 of a second — a definition that fixes the speed of light exactly and makes the metre reproducible in any laboratory.
As with mass, the imperial-to-metric factors are exact rather than approximate. The international yard was defined as exactly 0.9144 metres in 1959, from which every other imperial length follows.
| From | To | Multiply by |
|---|---|---|
| Inches | Centimetres | 2.54 |
| Centimetres | Inches | 0.393701 |
| Feet | Metres | 0.3048 |
| Metres | Feet | 3.28084 |
| Miles | Kilometres | 1.609344 |
| Kilometres | Miles | 0.621371 |
| Nautical miles | Kilometres | 1.852 |
| Yards | Metres | 0.9144 |
Unlike every other unit here, the nautical mile has a geographic basis: it was defined as one minute of arc of latitude along a meridian, which makes navigation arithmetic straightforward — one degree of latitude is 60 nautical miles anywhere on Earth. It is now fixed at exactly 1,852 metres, since the Earth is not a perfect sphere and the arc length varies slightly with latitude. It remains standard in marine and air navigation, and the knot is one nautical mile per hour.
The United States maintained a second, very slightly different foot for land surveying — the US survey foot, which is 1200/3937 metres rather than exactly 0.3048. The difference is about two parts per million, which is invisible over a room and amounts to roughly 3 mm per kilometre. Across a state-scale coordinate system it becomes metres of error, which is why the US formally retired the survey foot at the end of 2022 in favour of the international foot. Historical survey data and older state plane coordinates may still use it.
A related caution: fractional inches. Imperial construction and engineering work in halves, quarters, eighths and sixteenths rather than decimals. Three and three-eighths inches is 3.375 in, not 3.38 or 3.3, and misreading a fraction as a decimal is a classic and costly workshop error.
Rounding decisions that seem harmless at small scale can matter at large. Converting 0.5 inches to 1.3 cm rather than 1.27 cm is a 2% error, which will not fit a machined part. Over long distances the opposite problem appears — quoting a marathon as 42 km rather than 42.195 km loses nearly 200 metres.
The general rule is to keep at least as many significant figures as your original measurement carried, convert once from the original rather than chaining conversions, and only round at the final step. If a specification gives a tolerance, work to a precision comfortably finer than that tolerance.
Exactly 2.54. This is a definition rather than a measurement — the international inch was fixed at 2.54 cm by agreement in 1959, so the conversion carries no inherent error.
Multiply by 0.6, which is accurate to within about 3.5%. Taking five-eighths is closer still. For a rough sense of highway speeds, 100 km/h is 62 mph and 50 km/h is 31 mph.
It was defined as one minute of latitude, which makes navigation calculations simple — one degree of latitude equals 60 nautical miles. It is now fixed at exactly 1,852 metres, about 15% longer than a statute mile.
A slightly different foot, 1200/3937 metres, used historically for American land surveying. It differs from the international foot by two parts per million, which is negligible in a building but significant across a state coordinate system. It was officially retired at the end of 2022.
Convert the fraction to a decimal first: an eighth is 0.125, a quarter 0.25, three-eighths 0.375, five-eighths 0.625. So 5 and 3/8 inches is 5.375 in, which is 13.65 cm. Do not read the fraction as a decimal.