Convert mm to in

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.

An inch has been exactly 25.4 mm since 1959, when six countries agreed to stop using slightly different ones. Before that a US inch and a UK inch differed in the seventh significant figure: 25.400 05 mm against 25.400 00.

The factor is 0.03937, and almost nobody carries that around. Rounded to 0.039 it is off by 0.94 % — 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 inch 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 25.4 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.

The number this page gives you is a decimal inch, and no fractional rule, no imperial drill index and no woodworking plan is marked that way. 25 mm is 0.98425 in, and the thing to do with that is multiply by 64 and round: 62.99 becomes 63/64. The fraction is 0.984375 in, which sits three thousandths of a millimetre from where you started — inside the reading error of any rule you would use it on.

The same step works at any denominator. Multiply by 16 for a rule marked in sixteenths, by 32 for a finer one, by 64 for a machinist's scale, and round to a whole. 10 mm is 0.3937 in, which is 25.2 sixty-fourths, so 25/64 — and the leftover fifth of a sixty-fourth is 0.003 in, worth knowing about if it is a fit and worth ignoring if it is a shelf.

Below about a millimetre nobody in an imperial shop speaks in inches at all; they speak in thousandths. A thou is exactly 0.0254 mm, so 0.1 mm is 3.94 thou, 0.05 mm is a shade under 2, and 0.025 mm is 1. Going up, a millimetre is 39.37 thou, which is the number to remember because it makes every small metric figure a one-step multiplication.

The word "mil" means the same thing in American engineering and something else entirely elsewhere — in parts of Europe it has been used for a millimetre, and in plastics film thickness both conventions circulate. If a spec says "2 mil" and the source is American, it is 0.0508 mm. If the source is not, ask, because the two readings differ by a factor of forty.

A dimension on a metric drawing usually arrives with a band around it, and the band converts with the same exact factor as the dimension. 40 ±0.1 mm becomes 1.5748 ±0.00394 in, which in shop language is "one point five seven five, plus or minus four thou". The nominal and the limits should be converted separately and then checked against each other, because rounding them independently can put the stated nominal outside its own band.

Round tolerances outward. A ±0.1 mm band written as ±0.003 in is 24 per cent tighter than the designer specified, which turns a part that was economical into one that is not, and does it invisibly. The opposite mistake is worse in a different way: widening a fit tolerance to a convenient fraction produces parts that pass inspection and do not assemble.

The near-misses between the two systems are the expensive part of this conversion. M6 is 6 mm and a quarter inch is 6.35; M10 is 10 mm and three-eighths is 9.525; M12 is 12 and a half inch is 12.7. Each pair is within about six per cent, close enough that the parts look interchangeable in a drawer and far enough apart that they are not — and the thread pitches differ regardless, so even a matched diameter will not run.

Drill sizes have the same trap in a finer form. A 6 mm drill is 0.2362 in, which falls between the imperial letter sizes; the nearest fractional bit at 15/64 is 5.953 mm and the next at 1/4 is 6.35. Drilling a 6 mm clearance hole with a quarter-inch bit gives 0.35 mm of slop, which is fine for a bracket and not fine for a dowel or a bearing seat.

Metric sheet and plate are sold by thickness in millimetres, which converts cleanly: 1 mm is 0.0394 in, 1.5 mm is 0.0591, 3 mm is 0.1181. Imperial stock is often sold by gauge instead, and gauge is not a unit — it is a table, and there is more than one of them. Sheet steel gauge, aluminium gauge and wire gauge give different thicknesses for the same number.

That makes the conversion one-directional in practice. Take a millimetre thickness to a decimal inch and then look up which gauge is nearest, rather than trying to convert a gauge number to millimetres arithmetically. 16 gauge sheet steel is 1.519 mm and 16 gauge aluminium is 1.290 mm, an 18 per cent difference behind an identical label.

A drawing dimensioned in whole millimetres is making a claim about precision, and the claim is roughly half a millimetre either way unless a tolerance says otherwise. That is 0.020 in. Converting 47 mm to 1.850394 in and machining to six places is inventing four digits the drawing never had, and it costs real money in setup and inspection time.

The rule that survives contact with a shop is to keep one more decimal place than the source justified and stop. Whole millimetres go to three decimal inches; tenths of a millimetre go to four. Where the millimetre figure came off a caliper reading to 0.01 mm, four places is honest and five is not — a caliper repeatable to 0.01 mm is repeatable to 0.0004 in and no further.

Because 25.4 is exact, every correct converter agrees to as many digits as you care to compare, and any disagreement is a rounding choice rather than a difference of opinion about the unit. The commonest source is a converter that rounds the factor to 0.03937 before multiplying instead of dividing by 25.4 — an error of about two parts in a million, invisible at 50 mm and worth a thousandth of an inch at 20 metres of stock.

The second source is the fraction. Two tools can both be right and disagree because one snapped to the nearest 32nd and the other to the nearest 64th. 33 mm is 1.29921 in, which is 1 and 19/64 at one resolution and 1 and 5/16 at the other — 0.0156 in apart. Neither is wrong; they answered different questions, and only the drawing knows which one was asked.

Some millimetre dimensions are converted inches wearing metric clothes, and they are easy to spot once you know the numbers. 25.4, 12.7, 6.35, 3.175, 19.05 and 38.1 are an inch, a half, a quarter, an eighth, three-quarters and an inch and a half. A drawing full of them was authored in inches and translated, and converting it back gives round fractions rather than the awkward decimals a genuinely metric drawing produces.

Knowing which kind of drawing you have changes what to do with it. A soft-metric dimension should go back to its original fraction, because that is the size the tooling, the stock and the fasteners exist in — a 6.35 mm hole is a quarter-inch hole and should be drilled with a quarter-inch bit. A hard-metric 6 mm hole is a different hole, and rounding one to the other in either direction is how a part ends up with the wrong clearance.