Convert ft to cm

A foot is twelve inches, and it is the unit heights and room sizes are quoted in across the United States. Aviation quotes altitude in feet almost everywhere, including in countries that are metric for everything else; China is the exception, where controllers clear aircraft in metres.

Centimetres are the everyday metric length — body measurements, paper, furniture — and they are the one metric unit that has no direct customary counterpart, which is why so much converting happens through them.

The factor is 30.48, and almost nobody carries that around. Rounded to 30 it is off by 1.6 % — which stays invisible on small numbers and turns into a whole unit somewhere around 100 ft.

That is the number worth knowing before you round: not the error itself, but where it stops being ignorable. Below that point the shorter factor is the sensible one; above it, use the field above, which never rounds until it prints.

The foot has been defined in metric terms since the international yard and pound agreement of 1959, and in this page's units that comes out at 30.48 cm to one. The definition is exact by stipulation — it was not measured, and it cannot be refined by measuring it better. The decimal above is the whole of it, with nothing rounded away at the end.

Before that agreement the US and the British versions differed slightly, and old survey figures still carry the older definition. For anything outside land surveying the difference is far below the precision anybody is working to.

A height written 5'10" gets typed as 5.10 more often than any other conversion error on this page produces, and the result is 155.45 cm instead of 177.8. Twenty-two centimetres is the difference between an average adult and a child, and it passes every validation a form is likely to apply because it is a plausible number in the right range for somebody.

The reason is that feet and inches is a compound notation, not a decimal one. The part after the mark counts in twelfths, so five foot ten is 5 + 10/12 = 5.8333 feet. Anybody converting by hand should never produce a decimal foot at all: go to inches, multiply by 2.54, and the ambiguity has nowhere to hide.

Multiply the feet by twelve, add the inches, multiply by 2.54. Five foot four is 64 inches and 162.56 cm; five foot seven is 67 inches and 170.18 cm; six foot two is 74 inches and 187.96 cm. The intermediate number is worth writing down because it is checkable — a human height in inches lands between about 55 and 80, and anything outside that band means the first step went wrong.

Doing it in the other order works arithmetically and fails in practice. Converting the feet and the inches separately and adding — 5 × 30.48 plus 10 × 2.54 — gives the same 177.8, but it is two multiplications and two chances to drop a digit, and it hides the sanity check that the inch total provides.

Heights in feet and inches are recorded to the nearest inch, and an inch is 2.54 cm. So a passport saying 5 ft 9 in is asserting a range about 2.5 cm wide, and converting it to 175.26 cm asserts a hundredth of a centimetre from a figure that never had one. On a form the honest answer is the whole centimetre: 175.

This is why a converted height and a measured one disagree. Somebody measured at 176 cm in a clinic and recorded as 5 ft 9 in on a licence is both, because 176 cm is 5 ft 9.3 in and the licence rounded down. The mismatch is a rounding artefact and not a discrepancy worth explaining to anybody — but it is worth knowing about before a form asks you to reconcile two documents.

Clinical practice is metric almost everywhere including the United States, so a height given in feet has to arrive as one metric number. Most intake forms want centimetres, and most calculations behind them want metres. A BMI formula takes kilograms over metres squared, so 5 ft 10 in has to become 1.778 m and not 177.8 — entering the centimetre figure gives a BMI ten thousand times too small, and it will be accepted silently.

Paediatric growth charts have the same shape and a sharper consequence, because a child's height is plotted against a percentile curve where a centimetre moves the reading. WHO and CDC charts are published in centimetres; a height converted from feet and inches and rounded to the nearest inch first carries a 2.5 cm quantisation into a chart whose bands are narrower than that. Measure in centimetres if the chart is in centimetres.

Take the total inches, multiply by two and a half, then add a bit under two per cent. Seventy inches is 175, plus 2.8 gives 177.8 — which happens to be exactly right, because 2.54 is 2.5 plus 1.6 per cent. Skipping the correction leaves you about 3 cm short at adult heights, which is close enough to sanity-check a form and not close enough to fill one in.

The other direction is worth having for reading metric heights back: divide centimetres by 2.5 and take about 1.6 per cent off to get inches, then divide by twelve. 180 cm gives 72 minus 1.1, so about 70.9 inches, which is five foot eleven. The exact answer is 70.87 in, and the estimate was never going to be wrong by a whole inch.

The United States records and speaks height in feet and inches everywhere — licences, medical intake, sports rosters, dating profiles — while its clinical measurements are metric behind the form. The United Kingdom is the split case: heights are spoken in feet and inches and written in centimetres, in the same way weight is spoken in stone and written in kilograms. Ireland behaves the same way.

Everywhere else the unit is essentially absent from bodies and present in exactly one place: aviation altitude, which is quoted in feet across most of the world including thoroughly metric countries — China and North Korea are the exceptions, filing flight levels in metres. That is why a page about converting feet is usually about one of two entirely unrelated things, and why a height conversion and an altitude conversion share a factor and nothing else.

The international foot is exactly 0.3048 metres, which follows from the yard being exactly 0.9144 m under the 1959 international yard and pound agreement. Nothing about the number was measured; six countries agreed on it, and the inch, foot, yard and mile all fall out of the same definition. So a height conversion carries no uncertainty from the units at all — only from the height.

The United States kept a second foot alive for another sixty-three years. The US survey foot was 1200/3937 of a metre, longer than the international one by about two parts in a million, and it existed because a century of state plane coordinates had been computed in it. It was formally retired at the end of 2022. Two parts per million is 2 mm over a kilometre and about four thousandths of a millimetre on a human height, which is a good illustration of why nobody converting a height ever needed to know it.

Spinal discs compress under load through the day and rehydrate overnight, so an adult measured first thing in the morning is measurably taller than the same adult measured in the evening — commonly by a centimetre or more. That variation is comparable to the 2.54 cm quantisation an imperial height already carries, which means the two largest sources of disagreement between a converted height and a measured one have nothing to do with the arithmetic.

The practical consequence is to stop chasing the last centimetre. If a form wants a height, give the whole centimetre from a conversion of the feet and inches you normally state, and do not try to reconcile it against a clinic measurement taken at a different hour with shoes off and a different stadiometer. Where a height genuinely has to be precise — a medical study, a fitting, a records claim — it is measured under stated conditions rather than converted from anything.