How this instrument works
Feet are not measured against anything; they are declared. Effective 1 July 1959, national standards laboratories of six countries — United States, United Kingdom, Canada, Australia, New Zealand and South Africa — adopted one international yard of 0.9144 m, and a third of that yard is our foot at 0.3048 m. Before that agreement each country carried its own answer. Britain's descended from a bronze bar cast in 1845 that was quietly shrinking by roughly one part per million each century; America's came from an 1893 Mendenhall Order, which had already tied yards to metres at 3600⁄3937.
Written as a fraction, 0.3048 is 381⁄1250 — it terminates cleanly in decimal and counts as exact in strict metrological terms: no uncertainty, no error bar, nothing to propagate through downstream arithmetic. Dependency also runs one way only. Since 1983 metres have been fixed by light speed at 299 792 458 m/s, so foot length now traces back through a treaty to a clock and a beam of light. Nothing anywhere defines metres in terms of imperial units.
Both scales remain in daily working use, often in different rooms of one building. Aviation is loudest among holdouts: altitudes and flight levels get reported in ft almost everywhere, including across metric Europe, with China as notable exception, still assigning cruising levels in metres — Russia moved its upper airspace over to ft during 2011. American builders order lumber, electrical conduit and drywall in whole ft; climbers and free divers argue about identical routes and depths on two incompatible scales. Everywhere else — engineering drawings, athletics, cartography, medicine — metres and millimetres rule alone.
- Type your measurement into Feet (ft); that field opens with 10 already loaded as a reference value.
- Read your answer on Metres (m), which recomputes on every keystroke and carries six decimal places.
- For feet-and-inches figures, make those inches decimal first: divide by 12 and add, so 6 ft 3 in becomes 6.25.
- Going backwards? Divide your metric figure by 0.3048 — 100 m is 328.083990 ft — or nudge Feet (ft) until Metres (m) matches.
- Round only your finished answer. Any length rounded before conversion carries its error straight through untouched.
Worked example — one 10 ft stick of conduit
American electrical conduit sells in 10 ft sticks, and any shop drawing crossing to Europe has to say what that means in metric. Multiply 10 by 0.3048 and your answer is 3.048 m exactly — 3048 mm, landing on one whole millimetre with nothing left over.
That tidiness is not luck. Since 0.3048 m equals 304.8 mm, every whole number of ft converts to a length ending on one tenth of a millimetre, and every tenth ft lands squarely on whole millimetres. Specifying 3.05 m instead adds 2 mm per stick — trivial once, less so when forty get coupled end to end and your run finishes 80 mm long.
Questions
Is 0.3048 exact, or has it been rounded?
Exact. That 1959 international yard and pound agreement fixed one yard at precisely 0.9144 m, which makes one foot precisely 0.3048 m and one inch precisely 25.4 mm. These are definitions rather than measurements, so they carry no uncertainty and no error bar. Any rounding you spot in a converted figure comes from how many digits sit on display, never from 0.3048 itself.
Does a US survey foot give different answers?
Slightly, and that is precisely why it was retired. Survey feet kept an older 1200⁄3937 m value, roughly 0.304800610 m — about two parts per million longer than international feet. NIST and NOAA withdrew that unit from use on 31 December 2022. Over 10 ft this discrepancy stays under one hundredth of a millimetre, but on state plane coordinate grids whose values run into millions it grows to several ft, so legacy survey data is worth checking before you convert it here.
How do I convert a height such as 5 ft 9 in?
Turn those inches into a decimal fraction of one foot first, then run your whole figure through. Nine inches is 9 ÷ 12 = 0.75, so height becomes 5.75 ft, and 5.75 × 0.3048 = 1.7526 m — that 1.75 m which would appear on your passport. One classic slip treats 5 ft 9 in as 5.9, which returns 1.798 m and adds nearly five centimetres to your subject.
Why do pilots still use feet if aviation is international?
Because altimeter scales were standardised on ft while British and American manufacturers dominated this industry, and re-marking every instrument, chart and procedure worldwide would cost far more than any inconsistency does. Vertical separation minima are written in ft, flight levels are ft in hundreds, and controllers across metric Europe issue them without hesitating. China stands out as an exception, assigning cruising levels in metres instead.
What is a decent approximation to do in your head?
Take one third and shave a twelfth off it. Thirds overshoot by roughly nine per cent, so 60 ft becomes 20, minus about 1.7, giving 18.3 m against 18.288 m exactly. Reverse direction is easier still, since one metre runs just over 3.28 ft — treble it and add a little. Keep these for sanity checks; anything that gets built, billed or filed deserves full precision.
Why does this converter show six decimal places?
Because 0.3048 is exact, extra digits cost nothing and occasionally matter. Any whole number of ft terminates within four decimal places — 7 ft is 2.1336 m — and that spare room handles fractional entries such as 12.375 ft without silently truncating them. Round your display down to whatever drawing tolerance demands; arithmetic underneath is not rounding on your behalf.