How this instrument works
A foot is 30.48 centimetres exactly, and “exactly” is doing real work in that sentence. In July 1959, standards laboratories from six English-speaking nations — America, Britain, Canada, Australia, New Zealand and South Africa — signed an international yard and pound agreement fixing one yard at 0.9144 metre. Divide by three for one foot, 0.3048 metre, then multiply by 100 to land on centimetres. Nothing along that chain is measured or approximated, so this factor carries no uncertainty whatever.
Centimetres arrived by a different road. Revolutionary France defined its metre in 1795, and Latin centi- cut that length into hundredths; for roughly eighty years afterwards it ranked as a base unit, anchoring CGS physics beside gram and second. SI eventually promoted metres instead, yet centimetres kept every human-scale job — body heights on identity cards, waistbands and hem lengths, cabin baggage limits, furniture listings.
Feet survive where older habits dug in deepest. Every airliner aloft reports altitude in feet, whatever country lies beneath it; American lumber, ceiling heights and lot lines are quoted in feet; British and American people still give their own height that way. So this conversion tends to surface at a border — a visa form demanding centimetres, a European sofa spec beside an American living room, a basketball roster printing 6 ft 7 in where FIBA would write 201 cm. A standard interior door, 6 ft 8 in, is 203.2 cm; a plywood sheet's long edge, 8 ft, is 243.84 cm.
- Type your measurement into the Feet (ft) field — it opens at 6, roughly a tall adult's height.
- Working from feet and inches? Divide those inches by 12 and add them on: 5 ft 9 in is 5.75 ft, not 5.9 ft.
- Read your answer on the Centimetres (cm) line, which recomputes on every keystroke.
- To reverse this conversion, divide any centimetre figure by 30.48 — 170 cm comes back as 5.577 ft, or 5 ft 6.9 in.
Worked example — a six-foot height on a metric form
A clinic in Lyon hands you a form with one box for height in centimetres, and you have only ever known yourself as six feet tall. Enter 6 in the Feet (ft) field and the Centimetres (cm) line reads 182.88. That is your whole answer: 6 × 30.48 = 182.88, a decimal that stops dead after two places because its defining factor stops after two places.
Sanity-check it as a carpenter would. Thirty times six is 180, so the true figure has to sit just above 180 — that mental shortcut runs about 1.6 per cent low, which amounts to 2.88 cm here. And if you are really 6 ft 1 in, convert that inch first: 1 ÷ 12 = 0.0833 ft, giving 6.0833 ft, which lands on 185.42 cm.
Questions
Is 30.48 an exact factor or a rounded one?
Exact. That 1959 international yard and pound agreement defines one yard as 0.9144 metre, making one foot 0.3048 metre and therefore 30.48 centimetres, with no uncertainty attached. Any rounding you see in a result comes from how many decimals are displayed, never from this factor. Run it backwards and things get messier: 1 cm = 25⁄762 ft = 0.032808398950…, a repeating decimal that is nevertheless an exact fraction.
How do I convert a height given as 5 ft 11 in?
Turn those inches into a decimal fraction of a foot before converting: divide by 12, so 11 in = 0.9167 ft and your height is 5.9167 ft, which this sheet reads as 180.34 cm. Typing 5.11 is a classic mistake and lands you at 155.75 cm, more than 24 cm short. If you would rather skip that division, multiply total inches by 2.54 instead — 71 in × 2.54 = 180.34 cm, an identical answer by a shorter road.
How many decimal places should I keep?
Match your input. Height rounded to whole inches is good to no better than 1.3 cm, so 182.88 cm claims far more precision than any tape measure gave you — write 183 cm on a form and stop there. Machining and survey work differ: exactness of 30.48 is what lets you carry full digits to the final step without compounding error. Six decimals are shown so you can choose where to cut, rather than inheriting somebody else's rounding.
Do American and British feet give the same answer?
They do now. Before 1959 they did not quite: America had defined its foot through an 1893 Mendenhall Order as 1200⁄3937 metre, about 30.480061 cm, while Britain worked from a slightly different physical yard bar. That agreement retired both for ordinary use. An older American value lived on as the US survey foot inside state plane coordinate systems until NIST and NOAA deprecated it in late 2022; it runs two parts per million long — nothing on a door frame, metres across a state.
Why do aircraft still report altitude in feet?
Because safe separation depends on every crew reading one scale, and the world fleet, its altimeters and its radio phraseology standardised on feet early. ICAO permits either system: China and a few neighbours fly metric levels, while Russia moved its upper levels to feet in 2011. Flight level 350 means roughly 35,000 ft, or 1,066,800 cm — a number nobody would ever say aloud, which is a fair illustration of why centimetres stay closer to ground level.
Has a foot always been 30.48 cm?
Far from it. This unit began as a body measure and drifted by region: the Roman pes sat near 29.6 cm, the pied du roi of pre-revolutionary Paris ran to about 32.48 cm, and German and Nordic towns each kept their own. Shared ancestry is why that name survived translation while its length did not. Only one treaty-defined value, 30.48 cm, applies to anything you are likely to measure today.