The exact definitions behind every length conversion
Every factor on this page comes from the metre. The SI metre is defined from the speed of light, and the imperial units hang off it by agreement rather than by measurement. In 1959 the United States, United Kingdom, Canada, Australia, New Zealand and South Africa agreed that the international yard is exactly 0.9144 m. From that: 1 inch = 25.4 mm, 1 foot = 0.3048 m and 1 mile = 1609.344 m, all exact. So 6 feet is exactly 182.88 cm and the conversion contributes no rounding error of its own. The converter holds all ten units (mm, cm, m, km, in, ft, US survey foot, yd, mi, nmi) against the metre and shows the answer to seven significant digits.
Worked example
A height of 5 ft 10 in is 70 inches in total, and 70 × 0.0254 = 1.778 m. Enter the feet and inches separately and add them, since the tool takes one value in one unit at a time. For running distances, a 5 km race is 5000 / 1609.344 = 3.107 miles and a marathon of 42,195 m is 26.219 miles.
Quick reference
| Unit | In metres | Note |
|---|---|---|
| 1 inch | 0.0254 | exactly 25.4 mm |
| 1 foot | 0.3048 | 12 inches |
| 1 yard | 0.9144 | 3 feet |
| 1 mile | 1609.344 | 5,280 feet, 1,760 yards |
| 1 nautical mile | 1852 | 1.15078 statute miles |
Nautical miles and the survey foot
The nautical mile is not a land mile. It was fixed internationally at exactly 1,852 m in the interwar period, close to one minute of latitude, which is why charts and navigation use it. At sea and in the air it pairs with the knot; see the speed converter. Do not confuse 1 nmi (1.852 km) with the 1.609 km statute mile.
The US survey foot is a slightly different unit, defined as 1200/3937 m, about 0.3048006 m. It is two parts per million longer than the international foot. That is 0.6 m over a million feet, which matters for state-plane coordinates and old land surveys. The US standards agencies deprecated it at the end of 2022, and survey work now uses the international foot. The Foot in this converter is the international foot, and the US survey foot has its own entry in the list, so pick it when a legacy survey document says "US survey feet". For the full state-plane workflow, a dedicated geodetic tool is still the safer choice.
Common mistakes
- Entering 5.10 for "5 feet 10 inches". That is 5.10 feet (5 ft 1.2 in), not 5 ft 10 in.
- Treating a kilometre as 0.6 mile. The factor is 0.621371; for rough mental maths, 5 miles is about 8 km.
- Very small or very large results switch to scientific notation (below 0.000001 or above 1e15), so read the exponent.
The principle of choosing between the two systems is covered in metric vs imperial units, and the all-in-one unit converter switches categories without leaving the page.