How to convert a length
- Enter the value you want to convert.
- Choose the unit it is currently in.
- Choose the unit you want it in.
- Read the result, along with the conversion factor in both directions.
- Scan the list below to see the same length in every other unit.
What this converter covers
- Metric units from millimetres to kilometres.
- Imperial units from inches to miles.
- Nautical miles, which are defined separately and used in aviation and shipping.
- The conversion factor in both directions, so you can check the arithmetic.
- Every other unit shown alongside, which saves converting twice.
- Exact international definitions rather than rounded approximations.
The definitions are exact, which surprises people
Since 1959 an inch has been defined as exactly 25.4 millimetres. Not approximately, exactly. The international yard and pound agreement fixed the imperial units in terms of metric ones, which means every conversion between the two systems is exact rather than an approximation.
That is why a foot is exactly 0.3048 metres and a mile is exactly 1609.344 metres. The numbers look arbitrary because they are defined by agreement rather than derived from anything physical.
The metre, and how its definition changed
The metre was originally one ten-millionth of the distance from the equator to the North Pole, measured along a meridian through Paris. That definition depended on a survey that turned out to be slightly wrong, so it was replaced by a physical bar, then by a wavelength of krypton light.
Since 1983 it has been defined as the distance light travels in a vacuum in 1/299,792,458 of a second. That definition fixes the speed of light exactly and makes the metre reproducible in any properly equipped laboratory, which a platinum bar in a vault never was.
Nautical miles are a different kind of unit
- A nautical mile is 1,852 metres exactly, about 15 per cent longer than a statute mile.
- It was defined as one minute of latitude, which makes navigation arithmetic straightforward.
- A knot is one nautical mile per hour, which is why sea and air speeds use it.
The connection to latitude is the point: one minute of arc along a meridian is one nautical mile, so a chart distance converts directly into a position change. That relationship is why aviation and shipping have never switched to kilometres.
Survey feet and why they caused problems
The United States maintained a second definition, the survey foot, for land surveying, defined as 1200/3937 metres rather than exactly 0.3048. The difference is about two parts per million, which is nothing over a room and roughly 3 centimetres over 15 kilometres.
That was enough to cause real discrepancies in state plane coordinate systems, and the survey foot was formally retired at the end of 2022. If you are working with older surveying data, the distinction may still matter.
Practical conversions worth knowing
A metre is a little over three feet. A kilometre is about six tenths of a mile, so 100 km/h is roughly 62 mph. An inch is two and a half centimetres. Six feet is 1.83 metres. These approximations are close enough for conversation and wrong enough for engineering, which is what a converter is for.
For weights the weight converter covers the same ground, and the pixels to inches converter handles the screen-to-physical conversion that needs a resolution figure. Everything runs in your browser.