Height Converter

The number 2.54 is not an approximation

This tool converts a height between feet and inches, plain inches, and centimetres. It is not complicated and you do not need a page of explanation to use it. But there is one fact worth knowing, because it is genuinely unusual and it changes how much you should trust the answer.

An inch is exactly 2.54 centimetres. Not approximately. Exactly, by definition, with nothing rounded off.

That is a strange thing for a unit conversion to be. Most conversions between measuring systems are approximations, because the two units were defined independently by different people for different reasons and there is no reason on earth they should line up neatly. A pound is about 453.592 grams and the decimals keep going. A mile is a mess.

The inch is not like that, and the reason is that somebody sat down and decided it would not be. Until the middle of the twentieth century the inch genuinely was a slightly awkward number, and worse, it was a slightly different awkward number depending on which country you were in: the American and British inches did not quite agree. For most purposes nobody noticed. For precision engineering across an ocean, it was a real problem.

So in 1959 the English-speaking countries came to an agreement and simply defined the thing into existence: the yard would be exactly 0.9144 metres, which makes the inch exactly 2.54 centimetres, and everyone would use the same one. The imperial system was, quietly, given a metric foundation. Every inch you have measured since is defined in terms of the metre.

Which means the conversion this tool performs is not an estimate. If it says 5 foot 10 is 177.8 cm, that is not close to the answer, it is the answer, with nothing lost.

The sums, if you want to do them yourself

Feet and inches are a base-12 system riding on a base-10 one, which is why people find them awkward. The trick is to convert everything to plain inches first, then get out.

Feet and inches to centimetres. Multiply the feet by 12, add the loose inches, then multiply by 2.54.

5 foot 10 → (5 × 12) + 10 = 70 inches
70 × 2.54 = 177.8 cm

Centimetres to feet and inches. Divide by 2.54 to get total inches. The whole part divided by 12 gives your feet, and the remainder is your inches.

170 cm ÷ 2.54 = 66.93 inches
66.93 ÷ 12 = 5, remainder 6.93
So 170 cm ≈ 5 foot 6.9, which people call 5 foot 7.

A couple of landmarks worth carrying in your head, since they cover most of the range you will ever need:

The rough mental version, good to about a centimetre across normal human heights: every inch is two and a half centimetres, and 6 foot is a bit under 183.

Where the error actually comes from

Given the conversion is exact, anything wrong with your answer came from somewhere else. Two places, in practice.

Rounding to the nearest inch. This is the big one, and it is baked into how people speak. Nobody says they are 5 foot 6.93. They say 5 foot 7. So "5 foot 7" is not a height, it is a bucket about 2.5 cm wide, holding everyone from roughly 169.5 to 172 cm. Convert 5 foot 7 to centimetres and you get 170.2, and that is only the middle of the bucket you were actually handed. This is why converting to centimetres and back can appear to lose you a few millimetres. Nothing was lost in the arithmetic. It was lost when someone rounded, before the number ever reached the tool.

The practical upshot: if you need your height in centimetres for something that matters, measure it in centimetres. Do not measure in inches, round, and convert. Convert only when you were given a centimetre figure in the first place.

Measuring badly. Which swamps everything above. People are genuinely up to a centimetre taller in the morning than at night, because the discs in your spine compress across the day under your own weight, and recover when you lie down. Shoes obviously matter. So does tipping your chin up to be helpful, which adds a centimetre and which almost everybody does. Against errors of that size, arguing about a rounded inch is missing the point.

If you want the number to be right: shoes off, heels and back against a wall, head level, something flat resting on your head at a right angle to the wall, and measure at a consistent time of day. Then the exact conversion has something worth converting.

If you are converting a height in order to do something else with it, our BMI Calculator and Height Percentile Calculator both take either unit directly, so you can skip this step entirely.

Questions people ask

Is 2.54 rounded?

No. The inch has been defined as exactly 2.54 cm since the international agreement of 1959. It is a definition, not a measurement, so there is nothing after the 4.

How tall is 5 foot 7 in centimetres?

170.2 cm. Though bear in mind "5 foot 7" is itself a rounded figure covering about a 2.5 cm range, so the person in front of you is somewhere near 170, not exactly at it.

Why do I get a slightly different number converting back?

Rounding, not arithmetic. Going to feet and inches usually means rounding to the nearest inch, and that rounding is not recoverable. The conversion itself loses nothing.

Why are there 12 inches in a foot?

Because 12 divides neatly by 2, 3, 4 and 6, which is genuinely useful if you are dividing things up without a calculator. It is a worse system for arithmetic and a better one for carpentry, which tells you what it was for.

References

A note on the figures. The inch is defined as exactly 2.54 centimetres, following the international yard and pound agreement of 1959, under which the yard was defined as exactly 0.9144 metres. Before that agreement the inch differed very slightly between the United States and the United Kingdom. Every conversion this tool performs follows from that definition, so the arithmetic is exact and any imprecision in your answer comes from rounding or from measurement rather than from the conversion.