Length Converter
An inch is exactly 25.4 millimetres by international agreement, not 25 and a bit. Every conversion here uses the defined factor, so nothing drifts from the numbers people memorise.
Converted
39.370079inches
- millimetres
- 1,000.000000
- centimetres
- 100.000000
- metres
- 1.000000
- inches
- 39.370079
- feet
- 3.280840
- yards
- 1.093613
- Conversions use the defined factors rather than the rounded ones people memorise. An inch is exactly 25.4 millimetres by international agreement, not 25 and a bit, so these will not drift.
- Every unit in the family is shown, not just the one you asked for, because the next question is usually a different one.
About length conversion
Since 1959 the inch has been defined as exactly 25.4 millimetres, by agreement between the countries still using imperial units. Everything else follows: a foot is exactly 0.3048 metres, a yard exactly 0.9144. These are definitions rather than measurements, so they carry no error and never need revising.
That was not always true. Before 1959 the US and imperial inches differed very slightly, which mattered for precision engineering and survey work. A separate "US survey foot" persisted for land measurement until being retired in 2023.
The trap worth knowing is squaring. A square foot is not 0.3048 square metres — it is 0.3048 squared, which is 0.0929. Using the length factor on an area understates it by more than a factor of three, and it is one of the most common errors in ordering flooring. The area converter handles that properly.
For height, feet and inches are a pair rather than a decimal. Five feet eleven is not 5.11 feet — it is 5 plus 11/12, which is 5.917. Converting the decimal instead of the pair is the usual mistake.
What it works out
- Millimetres, centimetres, metres, inches, feet and yards
- Every unit shown at once, not just the one asked for
- Exact defined factors rather than rounded ones
The formula
Converted = Value × (metres per from-unit) ÷ (metres per to-unit)
Everything goes through metres. A centimetre is 0.01 metres and an inch is 0.0254, so 100 centimetres is 1 metre, and 1 divided by 0.0254 is 39.370 inches.
Routing through a single base unit rather than storing every pair means there are six factors rather than thirty, and no possibility of two routes disagreeing.
The factors are exact by definition. An inch is 25.4 millimetres precisely — not a measurement anyone took, but an agreement made in 1959 — so a conversion and its reverse always return the original.
A few worth carrying in your head: 2.54 centimetres to the inch, 30.48 to the foot, and a metre is 39.37 inches or 3.281 feet. A yard is 0.9144 metres, which is why a hundred metres is a little more than a hundred yards.
- Metre
- The base unit everything routes through.
- Inch
- Exactly 25.4 millimetres by definition since 1959.
- Foot
- Twelve inches, or exactly 0.3048 metres.
A worked example
One hundred centimetres in inches.
That works out to 39.370079 inches.
- millimetres
- 1,000.000000
- centimetres
- 100.000000
- metres
- 1.000000
- inches
- 39.370079
- feet
- 3.280840
- yards
- 1.093613
Questions
How many inches is 100 cm?
39.37 inches, which is 3 feet 3.37 inches. A metre is a little over three and a quarter feet.
How many cm are in an inch?
Exactly 2.54. It is a definition rather than a measurement, agreed internationally in 1959, so it carries no error.
How do I convert feet and inches to metres?
Convert the whole thing to inches first, then multiply by 0.0254. Five feet eleven is 71 inches, which is 1.803 metres. Treating it as 5.11 feet is the common mistake.
Why can I not use the length factor for area?
Because area is two dimensions. A square foot is 0.3048 squared, which is 0.0929 square metres — not 0.3048. Using the length factor understates an area by more than three times.
How many feet in a metre?
3.28084. The quick approximation of 3.3 is close enough for rough work and drifts noticeably over long distances.
What is a US survey foot?
A slightly different foot used for land surveying in the US, about two parts per million longer than the international one. It was retired in 2023 and only ever mattered over long distances.