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Length Converter
Length Converter
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How length conversion works
Every conversion on this page is exact, not approximate. Since the international yard and pound agreement of 1959, the inch has been defined as 25.4 millimetres, not measured as approximately that. The foot, yard and mile are then defined as multiples of the inch, so the whole imperial ladder is pinned to the metre by definition.
That means an inch-to-centimetre conversion has no measurement error in it at all. Any disagreement between two converters is rounding in the display, not uncertainty in the underlying factor.
Formulas
Inches to centimetres
cm = in × 2.54
Feet to metres
m = ft × 0.3048
Miles to kilometres
km = mi × 1.609344
Worked examples
| From | To | Result |
|---|---|---|
| 1 inch | cm | 2.54 cm |
| 6 foot | m | 1.8288 m |
| 5 mile | km | 8.04672 km |
| 100 m | foot | 328.084 foot |
| 30 cm | inch | 11.811 inch |
| 1 nautical mile | km | 1.852 km |
Results are computed with the same factors the converter above uses, rounded to six significant figures.
Common lengths
| cm | inch | foot |
|---|---|---|
| 1 | 0.393701 | 0.0328084 |
| 10 | 3.93701 | 0.328084 |
| 30 | 11.811 | 0.984252 |
| 50 | 19.685 | 1.64042 |
| 100 | 39.3701 | 3.28084 |
| 152.4 | 60 | 5 |
| 180 | 70.8661 | 5.90551 |
| 200 | 78.7402 | 6.56168 |
152.4 cm is exactly 5 feet, which is why it appears in height charts as a round imperial number and an awkward metric one.
Worth knowing
- 1 inch = 25.4 mm exactly. 1 foot = 12 inches = 304.8 mm exactly. 1 yard = 3 feet = 0.9144 m exactly.
- 1 mile = 1760 yards = 1,609.344 m exactly.
- A nautical mile is 1,852 m exactly, and is a different quantity entirely: it was defined as one minute of latitude, which is why aviation and shipping use it.
- The United States used two slightly different feet until the end of 2022. The US survey foot was 1200/3937 m, about 2 parts per million longer than the international foot. Over a room that is invisible; over a state plane survey it is metres.
Frequently asked questions
- Is the inch-to-centimetre conversion exact?
- Yes. One inch is defined as exactly 25.4 millimetres, so 2.54 centimetres is an exact figure rather than a rounded one. Every length conversion between metric and imperial units inherits that exactness.
- Why is a nautical mile longer than a statute mile?
- They measure different things. A statute mile is a land measure descended from the Roman mille passus. A nautical mile was defined as one minute of arc of latitude, so that a degree of latitude is 60 nautical miles, which makes chart navigation arithmetic trivial. It is 1,852 metres against the statute mile's 1,609.344.
- How tall is 180 cm in feet and inches?
- Just under 5 feet 11 inches. 180 centimetres is 70.87 inches, and 5 feet 11 inches is 71 inches, so the difference is about 3 millimetres.
- Why do two converters give slightly different answers?
- Almost always because they round at different points, not because they disagree on the factor. Converting through an intermediate unit and rounding twice compounds the error. The underlying definitions have been fixed and exact since 1959.