Length Converter
Metric and imperial, from nanometres to nautical miles. Enter one measurement and read it in every unit at once — on the exact defined factors, not on decimals someone rounded.
Convert a length
The measurement you have, in the unit chosen below.
The unit the measurement is already in.
The unit you want the answer in. Every other unit is listed underneath the result anyway.
Everything routes through the metre: 100 cm is 1 m, and one in is exactly 0.0254 m.
- Conversion factor
- 1 cm = 0.3937007874 in
- Reverse factor
- 1 in = 2.54 cm
- In metres
- 1 m
- Definition
- One inch is exactly 0.0254 m, fixed by the 1959 international yard and pound agreement.
| Unit | Symbol | Value |
|---|---|---|
| Nanometres | nm | 1,000,000,000 |
| Micrometres | µm | 1,000,000 |
| Millimetres | mm | 1,000 |
| Centimetres | cm | 100 |
| Metres | m | 1 |
| Kilometres | km | 0.001 |
| Mils (thou) | mil | 39,370.07874 |
| Inches | in | 39.37007874 |
| Feet | ft | 3.280839895 |
| Yards | yd | 1.093613298 |
| Furlongs | fur | 0.004970969538 |
| Miles | mi | 0.0006213711922 |
| Nautical miles | nmi | 0.0005399568035 |
Every conversion goes through the metre
Thirteen units could be connected by 169 direct factors, and a table that size cannot be kept consistent. Instead each unit is stored as one number — how many metres it is — and every conversion is two steps.
metres = value × f(from) • result = metres ÷ f(to)- f(u)
- Metres in one unit u — 0.0254 for the inch
- metres
- The measurement in the SI base unit
- result
- The measurement in the unit you asked for
What makes this exact rather than approximate is where the factors come from. These are definitions, not measurements:
- 1 yard = 0.9144 m exactly. Fixed by the international yard and pound agreement of 1 July 1959, signed by the national standards laboratories of the United States, United Kingdom, Canada, Australia, New Zealand and South Africa. Before it, an American inch and a British inch were very slightly different lengths.
- 1 foot = 0.3048 m, 1 inch = 0.0254 m. Both follow from the yard, being a third and a thirty-sixth of it.
- 1 mile = 1,609.344 m exactly — 5,280 feet. The furlong is an eighth of that, 201.168 m, and the mil is a thousandth of an inch, 0.0000254 m.
- 1 nautical mile = 1,852 m exactly, set at the 1929 International Extraordinary Hydrographic Conference in Monaco. The United States adopted it in 1954 and the United Kingdom in 1970.
- The metre itself is defined by the speed of light — the distance light travels in a vacuum in 1/299,792,458 of a second. Every other unit on this page is ultimately a multiple of that.
Results are rounded to twelve significant figures rather than to a fixed number of decimal places. A tool that spans nanometres to nautical miles has no single sensible decimal place — two decimals would turn a nanometre into zero. Significant figures are also what makes one mile come back as exactly 5,280 feet: 1609.344 ÷ 0.3048 in binary floating point is 5279.999999999999, and rounding to twelve figures restores the number the definition promises.
100 centimetres in inches
Step one: 100 × 0.01 = 1 metre. Step two: 1 ÷ 0.0254 = 39.37007874 inches. That is the whole calculation, and it is why the answer has no tidy ending — 1 ÷ 0.0254 is a repeating decimal, so any inch figure you have ever seen was cut off somewhere.
A longer one: 5 kilometres in miles. Step one gives 5 × 1000 = 5,000 m. Step two gives 5000 ÷ 1609.344 = 3.106855961 miles. Run it the other way and the 5 km parkrun is a shade over three and a tenth miles, which is the number to have in your head when a race is advertised in the unit you do not think in.
And a demonstration that the factors really are exact: convert 1 mile and read the table. Feet says 5,280, yards says 1,760, furlongs says 8. None of those are rounded results that happen to land on whole numbers — they are the definitions, arriving back intact.
What this converter assumes
- The foot, yard, inch and mile are the international ones defined in 1959. The US survey foot, 2 parts per million longer, is deliberately not offered — it was retired for new work at the end of 2022, and mixing the two without a stated coordinate system causes more errors than it prevents.
- A length is a magnitude, so negative values are rejected. If you are converting a difference between two measurements, convert its size and keep the sign yourself.
- Results carry twelve significant figures internally and are displayed to ten. Neither is a claim about your measurement — a tape measure read to the nearest millimetre is still only accurate to the nearest millimetre after conversion.
- Nothing here converts between length and anything else. Area and volume scale as the square and cube of a length factor, which is a different calculation and a different tool.
Length conversion FAQ
How many inches is 100 cm?
39.37007874 inches. 100 cm is exactly 1 metre, and an inch is exactly 0.0254 metres, so the answer is 1 ÷ 0.0254. It is not a rounded measurement — the only approximation is where you stop writing digits, because 1 ÷ 0.0254 never terminates.
Is an inch exactly 2.54 cm, or is that rounded?
Exactly. The international yard and pound agreement, signed by the national standards laboratories of the United States, United Kingdom, Canada, Australia, New Zealand and South Africa and effective 1 July 1959, defined the yard as exactly 0.9144 metres. That makes the foot exactly 0.3048 m and the inch exactly 0.0254 m — 2.54 cm with nothing after it.
Why do old survey figures disagree with this by a few parts per million?
Because the United States kept a second foot. Before 1959 the US inch came from the 1893 Mendenhall Order, which fixed the metre at 39.37 inches exactly — making the US foot 1200/3937 m, about 2 parts per million longer than the international foot. That value survived as the US survey foot for state plane coordinates. NIST and the National Geodetic Survey retired it for new work on 31 December 2022. Two parts per million is 3 mm in a mile and about 20 cm across 100 km, which is why it mattered for land records and never for anything else.
What is the difference between a mile and a nautical mile?
A statute mile is exactly 5,280 feet, or 1,609.344 m. A nautical mile is exactly 1,852 m — about 15% longer. The nautical mile was set to approximate one minute of arc of latitude, which is what makes it useful at sea and in the air: a degree of latitude is 60 nautical miles, near enough, anywhere on the globe.
What is a mil, and is it a millimetre?
No, and the collision causes real mistakes. A mil — also called a thou — is one thousandth of an inch, or 0.0254 mm. Circuit board traces, sheet metal, plastic film and wire insulation are all specified in mils. A millimetre is nearly forty times larger.
How long is a furlong?
Exactly 660 feet, 220 yards, or one eighth of a mile — 201.168 m. It comes from the length of a ploughed furrow in a medieval open field, and it survives today mainly in horse racing distances and in older land descriptions.
Sources and review notes
- NIST Special Publication 811 — Guide for the Use of the International System of Units (SI), including the exact conversion factors used here
- NIST — U.S. Survey Foot: revised unit conversion factors, and the 31 December 2022 deprecation
- BIPM — The International System of Units (SI) Brochure, for the definition of the metre
The factors on this page are defined values rather than measured ones, so they do not go stale: the inch has been exactly 0.0254 m since 1959 and the nautical mile exactly 1,852 m since 1929. The only thing that has changed recently is the retirement of the US survey foot, which this tool never offered.