Length and distance converter
Convert cm to inches, metres to feet, km to miles and 13 other length units instantly. Exact factors, formula shown, works offline.
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1 cm = 0.393700787402 in
Automatic formatting keeps up to 12 significant digits and switches to scientific notation only for numbers past 10¹⁸ or below 10⁻⁹. You can paste values with thousands separators, and fractions such as 3/4 or 1 1/2 are understood too.
in = cm ÷ 2.54
Your value: 1 ÷ 2.54 = 0.393700787402 in
Inch: Exactly 25.4 mm since 1959.
| cm | in |
|---|---|
| 1 | 0.393700787402 |
| 2 | 0.787401574803 |
| 3 | 1.1811023622 |
| 5 | 1.96850393701 |
| 10 | 3.93700787402 |
| 15 | 5.90551181102 |
| 20 | 7.87401574803 |
| 25 | 9.84251968504 |
| 50 | 19.6850393701 |
| 100 | 39.3700787402 |
| 500 | 196.850393701 |
| 1,000 | 393.700787402 |
| in | cm |
|---|---|
| 1 | 2.54 |
| 2 | 5.08 |
| 3 | 7.62 |
| 5 | 12.7 |
| 10 | 25.4 |
| 15 | 38.1 |
| 20 | 50.8 |
| 25 | 63.5 |
| 50 | 127 |
| 100 | 254 |
| 500 | 1,270 |
| 1,000 | 2,540 |
| Unit | Symbol | Value in m | Notes |
|---|---|---|---|
| MetreMetric | m | 1 m = 1 m | SI base unit of length. |
| KilometreMetric | km | 1 km = 1,000 m | — |
| CentimetreMetric | cm | 1 cm = 0.01 m | — |
| MillimetreMetric | mm | 1 mm = 0.001 m | — |
| Micrometre (micron)Metric | µm | 1 µm = 0.000001 m | — |
| NanometreMetric | nm | 1 nm = 0.000000001 m | — |
| MileImperial / US | mi | 1 mi = 1,609.344 m | Exactly 1,760 yards. |
| YardImperial / US | yd | 1 yd = 0.9144 m | Exactly 3 feet. |
| FootImperial / US | ft | 1 ft = 0.3048 m | Exactly 12 inches. |
| InchImperial / US | in | 1 in = 0.0254 m | Exactly 25.4 mm since 1959. |
| Nautical mileNautical & astronomical | nmi | 1 nmi = 1,852 m | One minute of latitude, exactly 1,852 m. |
| Astronomical unitNautical & astronomical | au | 1 au = 149,597,870,700 m | Earth–Sun distance, exact by definition. |
| Light-yearNautical & astronomical | ly | 1 ly = 9,460,730,472,580,000 m | Distance light travels in a Julian year. |
How to convert a length
- Type the number you have into either field — the other side updates on every keystroke, so you can work in whichever direction you happen to be thinking.
- Pick the units. The list is grouped into metric, imperial and US, and the nautical and astronomical units, so you are not scrolling past light-years to find millimetres.
- Use the swap button if you picked the pair the wrong way round. It keeps the equation intact rather than resetting it.
- Set the decimal places if you need a fixed number of digits — for drawings and cut lists, three decimals of a millimetre is usually more than enough.
Every pair in the "common conversions" list has its own link, so you can bookmark
?from=cm&to=in and land straight on the conversion you use most.
Which factors are exact and which are not
Most length conversions people worry about are exact, not approximations. Since the 1959 international yard and pound agreement, the inch is defined as exactly 25.4 mm, which makes the foot exactly 0.3048 m, the yard exactly 0.9144 m and the mile exactly 1,609.344 m. Nothing is rounded in those conversions — if you see 2.54 in a formula, that is the definition, not a measurement.
The nautical mile is exact too, at 1,852 m, because it was redefined away from its original "one minute of latitude" meaning to a round metric number. The astronomical unit was fixed at exactly 149,597,870,700 m in 2012, and the light-year follows from it because the speed of light is itself a defined constant. The only place rounding creeps in is your own choice of decimal places.
One trap worth knowing: the US survey foot, used in older American land records, is very slightly longer than the international foot — by two parts per million. It was officially retired at the end of 2022. Over a city block the difference is invisible; over a state boundary it is metres, which is why surveyors care. This converter uses the international foot throughout.
Reference: metric prefixes on the metre
| Unit | Symbol | Metres | Typical use |
|---|---|---|---|
| Kilometre | km | 1,000 | Road distances, running races |
| Metre | m | 1 | Rooms, fabric, height |
| Centimetre | cm | 0.01 | Body measurements, paper sizes |
| Millimetre | mm | 0.001 | Engineering drawings, timber, screws |
| Micrometre | µm | 0.000001 | Paint thickness, chip features, dust |
| Nanometre | nm | 0.000000001 | Wavelengths of light, semiconductors |
Quick mental conversions
- cm to inches: divide by 2.5 and take a little off — 30 cm is just under 12 in.
- metres to feet: multiply by 3 and add 10 %. 10 m becomes 33 ft, versus 32.8 exact.
- km to miles: multiply by 6 and drop a zero, so 8 km is about 4.8 mi (true value 4.97).
- miles to km: the Fibonacci trick — consecutive Fibonacci numbers are roughly a mile/km pair, so 5 mi ≈ 8 km and 8 mi ≈ 13 km.
All of these are estimates. The converter above is not: it carries full double precision and only rounds at the moment it prints, so what you copy out is the best available value at the precision you asked for.
Questions
How many centimetres are in an inch?
Exactly 2.54. The inch has been defined as precisely 25.4 mm by international agreement since 1959, so cm → in is a division by 2.54 with no rounding involved.
How do I convert metres to feet in my head?
Multiply by 3 and add ten per cent — 10 m becomes roughly 33 ft, against the true 32.81 ft. For anything that matters, use the exact factor of 3.280839895 shown in the formula panel.
Is a nautical mile the same as a mile?
No. A nautical mile is exactly 1,852 m — one minute of latitude — while a statute mile is 1,609.344 m. A nautical mile is about 15 % longer.
Can I convert astronomical distances?
Yes. Light-years and astronomical units are included, and results past 10¹⁸ automatically switch to scientific notation so nothing is truncated.
Does this send my numbers anywhere?
No. The whole conversion table and the arithmetic ship with the page and run in your browser, so the tool keeps working with the network switched off.