How this instrument works
Kilometres come out of legislation rather than measurement. France's metric nomenclature of 1795 attached kilo- to the metre for thousandfold multiples, and that metre had already been sized by Delambre and Méchain's seven-year survey of the Dunkirk-to-Barcelona arc: their quarter meridian was to run ten million metres, which puts a kilometre at one ten-thousandth of any journey from equator to pole. Since 1983 nothing so romantic is required — the metre is how far light travels in vacuum during 1⁄299,792,458 of one second.
Feet arrive from another direction entirely, by treaty instead of geodesy. An accord signed in 1959 pinned the international yard at 0.9144 m, which puts a foot at 0.3048 m with no uncertainty attached. Divide one thousand metres by that figure and out comes 1,250,000⁄381 ft — 381 being 3 × 127, with 127 prime, its reciprocal repeating in blocks of 42 digits, so 3280.839895013123359580052… never settles. This sheet carries 3280.83989501, that value rounded at eight decimal places, leaving it low by roughly a nanometre per kilometre: one part in a trillion.
Both units stay in service at once wherever ground rises. Mountaineers keep two vocabularies — fourteen summits above 8,000 m, which is 26,246.7 ft, and Colorado's fifty-odd fourteeners at 14,000 ft, which is 4.2672 km on the nose. Drillers across North America still log measured depth in feet, so 3 km of hole reads 9,842.5 ft on the morning report while that same well's directional plan is drawn in metres. Highway and rail chainage abroad counts kilometres; American stationing counts hundreds of feet, and both conventions meet in every imported set of drawings.
- Type your distance into Kilometres (km); it opens at 1, and its step arrows move in single metres.
- Read Feet (ft) directly below — recomputed on every keystroke, with no button to press.
- Holding a height in metres? Shift its decimal three places first, so 8,848 m becomes 8.848 km.
- Need feet back to kilometres? Divide by 3280.83989501, or nudge Kilometres (km) until Feet (ft) lands on your figure.
- Round at the very end. Any kilometre figure rounded early carries its error into your answer multiplied by 3,281.
Worked example — a tower drawn to break one kilometre
Jeddah Tower, rising on Saudi Arabia's Red Sea coast, was drawn to pass one kilometre — a round number in metric, deliberately awkward in feet. Enter 1 into Kilometres (km) and Feet (ft) returns 3280.83989501. Press coverage rounds that to 3,280 ft; Burj Khalifa's 2,717 ft trails it by more than five hundred.
Scale up and familiar figures appear. Everest at its long-standing 8,848 m — 8.848 km — returns 29,028.87 ft, which is where 29,029 ft on old expedition posters came from. Nepal and China's joint 2020 revision to 8,848.86 m moves it along to 29,031.69 ft, under three feet of difference. Every digit of that shift came out of re-surveying; this factor has not moved since 1959.
Questions
Is 3280.83989501 an exact figure?
No — it is a rounding, though very fine. Exact relation is 1 km = 1,250,000⁄381 ft, whose decimal expansion repeats forever in blocks of 42 digits: 3280.839895013123359580052… Cutting at eight decimal places leaves this converter low by about 3 × 10⁻⁹ ft per kilometre, near enough a nanometre. What sits underneath, 1 ft = 0.3048 m, is exact by definition and carries no uncertainty whatsoever.
What is the quickest way to do this in my head?
Treble it, then add nine and a third per cent. Your 5 km run is 15,000 ft plus roughly 1,400, giving 16,400 against 16,404.2 exactly. For tidier work, hold 3,281 ft per kilometre — high by a fifth of one foot each time. Read it downward instead and climbing figures fall out fast: 100 m of ascent is 328 ft, 10 m is 32.8 ft.
Do US survey feet change my answer over a kilometre?
Yes, by exactly 2 mm per kilometre — and they make for tidier arithmetic. Survey feet measured 1200⁄3937 m, putting a kilometre at exactly 3280⅓ of them, against 3280.8399 international feet. NIST and NOAA withdrew that unit after 31 December 2022. Over one kilometre nobody would notice 0.0066 ft, but state-plane coordinates run into millions of feet, where 2 ppm becomes several feet of position, so archived survey data deserves a check before you convert it.
Do imperial and US customary feet differ?
Not since 1959, when a six-nation accord settled the yard at 0.9144 m and swept away national feet which had disagreed in their sixth decimal. Both systems now use feet of 0.3048 m, so a kilometre gives 3280.84 ft in London and Denver alike. Divergence people remember belongs to volume: imperial gallons hold 4.546 L against 3.785 L for their American namesake, and imperial pints run 20 fluid ounces to America's 16. Length is safe; drinks are not.
Why does 5,280 give me the wrong answer?
Because 5,280 ft is a statute mile, not a kilometre. Substituting either for the other inflates your distance by about 61 per cent. One kilometre is 3,280.84 ft or 0.621371 mi; the mile is 1.609344 km exactly, since 1959 fixed it at 1609.344 m too. Whenever converted figures look too big by roughly three fifths, this swap is usually behind it.
Can I apply this factor to areas or volumes?
Only after raising it to a matching power. Areas want the square, so 1 km² is 10,763,910 ft², and volumes want the cube, roughly 3.53 × 10¹⁰ ft³ per cubic kilometre. Using this plain factor on square kilometres undercounts by a further factor of 3,281, a classic slip in land and reservoir figures. Gradients need no conversion at all: slopes of 1 in 20 stay 1 in 20, because ratios of two lengths have nothing left to convert.