How this instrument works
Millimetres come from decimal division: one thousand of them fit inside the metre, which itself has run on light since 1983, the year metrologists redefined it by declaring light's speed to be 299 792 458 metres per second and nothing else. Feet own no such anchor in physics. They are pegged to metric length instead — one inch means 25.4 mm precisely, so twelve inches mean 304.8 mm precisely — under an agreement six national standards bodies signed during 1959. Converting millimetres into feet is therefore arithmetic rather than measurement, and no uncertainty creeps in anywhere.
These two units share drawings every working day. Site offices across Britain, Ireland, Australia, New Zealand and South Africa dimension in whole millimetres, refusing centimetres and decimals alike, while catalogues, timber sizes and older habits still talk in feet. Watch what that mismatch does: plasterboard sold as 2400 × 1200 mm is not an 8 × 4 board, because 8 ft comes to 2438.4 mm, leaving each sheet short by 38.4 millimetres. Yet the standard British internal door at 1981 × 762 mm really is 6 ft 6 in by 2 ft 6 in, its width dead on and its height shaved by two tenths of one millimetre. British model railways took an even franker line and made millimetres per foot into their scale unit outright: 4 mm to the foot yields OO gauge at 1:76.2, and 7 mm to the foot yields O gauge at 1:43.5.
One caveat about digits. That multiplier, 0.00328083989501, is 5⁄1524 truncated at twelve significant figures; its true expansion runs 0.0032808398950131233… without ever settling. Truncation there costs under one part in one trillion, so one metre of steel lands wrong by roughly one millionth of one millimetre and no workshop will ever notice. What does bite people is the legacy US survey foot of 1200⁄3937 m, near 304.80061 mm, which NIST and NOAA retired at close of 2022 — two parts per million sounds trivial until state plane coordinates push that error out to whole metres.
- Type your dimension into Millimetres (mm); 1000 is preloaded, so this sheet opens on one metre.
- Read your answer on Feet (ft), recomputed at every keystroke.
- Drawing marked in metres or centimetres? Multiply by 1000 or by 10 first, because only millimetres are accepted here.
- Want feet and inches? Keep whole feet, then multiply what remains of that decimal by 12: 3.28084 ft becomes 3 ft 3.37 in.
- Running backwards, from feet into millimetres, multiply by 304.8 — an exact figure, so nothing is lost.
Worked example — one metre of skirting board
Our joiner in Manchester cuts skirting to drawings marked 1000 mm, then has to quote that run to an American supplier pricing trim by the foot. Leave Millimetres (mm) at its preloaded 1000 and Feet (ft) returns 3.28083989501 — one metre, carried to twelve figures.
Now translate that tail into workshop language. Multiply 0.28083989501 by 12 and you get 3.37007874 in, so your cut reads 3 ft 3.37 in. Reach for the nearest sixteenth and 3 ft 3⅜ in measures 1000.125 mm: one eighth of one millimetre proud of your pencil line, which is worth knowing before ordering material in bulk.
Questions
Is one foot exactly 304.8 millimetres?
Yes — exact by definition, not by anyone's measurement. That international yard and pound agreement concluded during 1959 fixed one inch at 25.4 mm, and twelve of those give 304.8 mm. Nothing along that chain was measured or estimated, so converting between millimetres and feet carries zero uncertainty; whatever rounding you meet happens only in how many decimals your display can hold.
Why does 0.00328083989501 never finish?
Because it is one fraction in disguise. One millimetre equals 5⁄1524 of one foot, and 1524 factors into 2 × 2 × 3 × 127; that 3 and that 127 divide no power of ten, so decimal digits repeat forever at 0.0032808398950131233… Twelve significant figures already sit far beyond anything your tape, your laser distance meter or your CNC machine can resolve.
Do American and British feet differ?
Not since 1959, when both became 304.8 mm. Earlier they genuinely differed: America worked from an 1893 Mendenhall Order inch of 1⁄39.37 m, about 25.4000508 mm, while Britain kept an inch tied to its bronze Imperial Standard Yard, near 25.399977 millimetres. Six standards bodies split that gap and adopted 25.4 mm flat. Surveying alone clung to its legacy foot, and NIST withdrew that one at close of 2022.
How do I turn decimal feet into feet and inches?
Keep your whole number and multiply what is left by 12. From 1000 mm this sheet gives 3.28083989501 ft: three whole feet, then 0.28083989501 × 12 = 3.37 in. Reading that decimal as though it were inches is the classic slip — 3.28 ft is not 3 ft 2.8 in, which would measure 985.5 mm, more than half an inch short of your metre.
Why will 2400 mm boards not suit 16 in stud centres?
Because metric building boards were sized as round metric numbers, never as converted imperial ones. Eight feet is 2438.4 mm, leaving each 2400 mm sheet short by 38.4 millimetres, and every 1200 mm width misses 4 ft by 19.2. Framing at 400 mm centres swallows this happily. Framing at 16 in centres, which equals 406.4 millimetres, does not: by your fourth stud, board and framing have drifted apart.
Is there any quick mental estimate?
Divide millimetres by 300, then shave off some. That turns 1000 mm into roughly 3.33 ft against 3.28 true, an overshoot near 1.6% and close enough for pacing out sites. Two anchors worth memorising: 305 mm sits within two tenths of one millimetre of the foot, and 3048 mm is ten feet on the nose.