Convert mm to ft

Millimetres are what engineering drawings and datasheets use, because a whole number of them is precise enough for almost anything made by machine and avoids a decimal point that can be lost in a photocopy.

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.

The factor is 0.003281, and almost nobody carries that around. Rounded to 0.0033 it is off by 0.58 % — which stays invisible on small numbers and turns into a whole unit somewhere around 1,000 mm.

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 304.8 mm 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 metric technical or architectural drawing uses millimetres for everything on it, and normally does not write the unit next to any dimension. A door opening is 926, a floor-to-floor height is 3200, a structural span is 12000. There is no switch to metres at some threshold, which is deliberate: one unit throughout means a dimension can never be misread as being in the other one, and a decimal point can never go missing in a photocopy.

That has a consequence for anybody converting. The figure you are handed may be four digits or six, and neither size tells you what part of the building it describes. Before dividing by 304.8, check the title block for the drawing scale and for a note giving the units — a drawing produced for a US client sometimes carries a second set of dimensions in brackets, and converting the bracketed figure is a mistake that is very hard to see afterwards.

Eight feet is 2,438.4 mm and four feet is 1,219.2. The European sheet of plasterboard or plywood is 2,400 by 1,200. That is 38.4 mm short on the long side and 19.2 mm short on the short side — roughly an inch and a half, and three quarters — and it is close enough to look like the same product and far enough to leave a visible gap at the end of a run.

It compounds along a wall. Four European sheets laid end to end cover 9,600 mm; four American sheets cover 9,753.6, a difference of just over 150 mm, which is a strip of board wider than a hand. If a drawing specifies a 2,400 module and the material arriving is sold by the eight-foot sheet, the setting-out has to be redrawn rather than adjusted at the last joint.

Timber framing in metric countries is set out at 400 or 600 mm centres. American framing is at 16 or 24 inches, which are 406.4 and 609.6 mm. Each pair looks like a rounding of the other and neither is. The metric module is the smaller one in both cases, so a metric layout puts more members into the same length of wall — and every sheet, insulation batt and service run sized for one module lands wrong on the other.

The arithmetic is worth doing once for the length you are working to. Over a twelve-metre wall, 400 mm centres give thirty bays and 406.4 mm centres give twenty-nine and a half; the half bay at the end is where the discrepancy shows up, and it shows up at the far end of the job rather than at the start. Insulation cut to 570 mm to friction-fit between 600 mm centres is loose between 24-inch ones by nearly ten millimetres.

The conversion has two halves and only the first is division. 3,650 mm divided by 304.8 is 11.975 feet; the whole feet are eleven, and 0.975 × 12 is 11.7 inches. So the figure is eleven foot eleven and about three quarters — which a fabricator can set out and 11.975 feet is not. Anything going to somebody with a tape measure has to arrive in that form.

How far to take the fraction depends on what will cut it. A sixteenth of an inch is 1.5875 mm and an eighth is 3.175, so rounding a drawing dimension to the nearest sixteenth costs at most eight tenths of a millimetre and to the nearest eighth at most 1.6 mm. For site carpentry that is below the tolerance of the work. For a machined part it is not, and those dimensions should go across in millimetres and be converted by the machine rather than by the drawing.

A US two-by-four is not two inches by four. It is dressed down to 1 1/2 by 3 1/2, which is 38.1 by 88.9 mm, and the nominal figure describes the rough sawn size before planing. The same applies through the range: a two-by-six is 38 by 140 mm, a one-by-four is 19 by 89. Converting the nominal number gives you a piece of timber that does not exist.

Sheet thicknesses behave the same way in both directions. A half-inch board is 12.7 mm and the metric board sold beside it is 12.5, and a 3/4-inch board is 19.05 against a metric 18 or 19. None of these differences matter to a wall and all of them matter to anything that has to slot into a rebate cut for the other one. Where a dimension is a name rather than a measurement, convert the actual size from the supplier’s data sheet and not the label.

Rounding direction is not a matter of taste once the part has to fit something. A hole, an opening or a clearance is rounded up, so the converted figure is never smaller than the drawing allowed; a shaft, a plug or anything that has to pass through is rounded down. Rounding both to the nearest value is what turns a specified clearance of one millimetre into an interference fit, and the fault is invisible on the paperwork because both numbers were correctly converted.

For anything with a stated tolerance, convert the limits rather than the nominal value. A dimension given as 250 ±0.5 mm becomes 9.8425 inches with limits at 9.8228 and 9.8622; converting only the 250 and then applying an imperial tolerance chosen by eye can widen or narrow the band without anybody deciding to. The tolerance is part of the dimension, and it converts with the same exact factor.

Some millimetre dimensions were never metric. 1,219 is four feet, 2,438 is eight, 610 is two, 305 is one, 25 is an inch and 19 is three quarters. When a drawing is full of numbers like these rather than of round hundreds, it was drawn in feet and inches and converted, and the imperial figures are the ones the designer chose. Converting them back should land on clean numbers, and if it does, you have found the original.

This matters because it tells you what is allowed to move. A native 1,200 mm dimension came from a metric module and can usually be adjusted to 1,219 without consequence; a 1,219 that means four feet cannot be trimmed to 1,200 without breaking the module it belongs to. Reading the drawing’s history off its own numbers is the cheapest check available before anything is ordered.