MM to Inches Converter

A millimetre figure almost always arrives attached to a job: a dimension read off an engineering drawing, a drill you are about to pick up, a plate thickness a stockist quoted, a thread you need to match. This mm to inches converter answers all three questions that job asks at once, giving decimal inches, the nearest fraction on a rule, and the one number nobody else prints, which is how far that fraction misses the true size in millimetres.

Why 25.4 is exact, and what that buys you

One inch is exactly 25.4 millimetres. That is a definition, not a measurement. The international yard and pound agreement, signed on 1 July 1959 by six nations (the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa), fixed the yard at exactly 0.9144 metres, and the inch follows from it at exactly 25.4 mm. Two engineering bodies had already jumped: ANSI adopted the 25.4 mm inch in 1933 and NACA followed in 1952, so the drawings were metric ready long before the treaty caught up.

Before 1959 the United States worked to an inch of roughly 25.4000508 mm. The difference is 0.0000508 mm per inch, invisible on a bench and significant only over survey distances, but it explains why very old American drawings and gauge blocks can disagree with a modern conversion in the last decimal place.

The practical consequence matters more than the history. Because the conversion factor is exact, the arithmetic adds no error at all. Divide by 25.4 to go from millimetres to inches, multiply by 25.4 to come back, and the answer is perfect until you round it. Every millimetre of error on this page is therefore a decision you made, not a limitation of the maths, and that is the reason the converter reports the size of it.

Decimal inches, fractional inches, and what each rounding costs

A decimal inch and a fractional inch are two different tools. A machine shop quotes thousandths of an inch, where one thou is 0.001 in, or 0.0254 mm. A tape measure or steel rule is marked in halves down to sixteenths, so a decimal answer has to be pushed onto the nearest mark before anyone can scribe it. Pushing it onto a mark throws away length, and the amount thrown away is fixed by the denominator you chose.

Every source that offers fractions warns that the answer will be slightly off. Here is what slightly means, in millimetres:

The worst case is always half a step, because the true length can fall exactly between two marks. Set that against the general tolerance a drawing already carries and the choice stops being a matter of taste. Under ISO 2768 medium class, a feature between 6 and 30 mm is allowed plus or minus 0.2 mm. A 1/32 in fraction can be 0.3969 mm out, so it is already outside general tolerance before anyone cuts anything, and 1/16 is nowhere near. Only 1/64, at 0.1984 mm, squeezes inside, and it uses almost the entire allowance doing it.

A worked example, with a rounding decision at every step

A drawing calls up a 38.5 mm width. The title block reads ISO 2768-m, so at that size the general tolerance is plus or minus 0.3 mm, because the medium class allows that from 30 to 120 mm. The shop works in inches. What number goes on the setup sheet?

Start with the exact division: 38.5 divided by 25.4 is 1.5157480315 in. Now look at the candidates. At four decimal places you get 1.5157 in, which converts back to 38.4988 mm and is out by about 0.0012 mm, so the rounding is invisible. At the nearest 1/64 you get 1 33/64 in, which is 1.515625 in or 38.4969 mm, out by 0.0031 mm and still comfortably inside. At the nearest 1/32 you get 1 17/32 in, which is 1.53125 in or 38.8938 mm, out by about 0.394 mm and therefore outside the band. At the nearest 1/16 you get a tidy looking 1 1/2 in, which is exactly 38.1 mm, out by 0.4 mm and also outside.

So two of the four candidates fail a tolerance that was never about the conversion at all. The setup sheet gets 1.5157 in, the marking out gets 1 33/64 in, and the neat 1 1/2 in never leaves the conversation. Notice also that the failures arrive without warning: 1 1/2 in looks like the friendliest number of the four and is the worst of them.

Millimetre to inch chart

Two bands, because small part work and panel work need different steps. Fractions are the nearest 1/64.

For reference at the far end, 1000 mm is 39.3700787402 in, and the reciprocal that drives the whole table is 1 mm equals 0.03937007874 in.

Inch fractions back to millimetres

The reverse direction is the one most converters bury, and it is the one you need when a supplier quotes imperial stock and the drawing is metric. Every value here is exact.

Near misses: threads, drills and clearance holes

The dangerous conversions are not the ones that come out wrong. They are the ones that come out almost right, where a metric part and an imperial part sit so close in diameter that they thread together for two turns and then jam. Diameter is the part that looks fine. Pitch is the part that destroys the thread.

The working rule is this: a difference of 0.1 mm or less is usually acceptable for a clearance hole, and never acceptable for tapping, reaming or a press fit. It is worth remembering why the two sets so rarely coincide. Imperial twist drills step in 1/64 in, which is 0.396875 mm, while metric drills step in 0.5 mm, so the two ladders interleave and almost never land on the same rung. Metric and imperial threads share nothing but a nominal diameter. The pitch and the thread angle differ, so a matching diameter is a coincidence, not a fit.

Sheet, plate and the gauge trap

Sheet thickness is where a correct conversion still produces a wrong part, because gauge is not a unit at all. It is a sheet number inherited from wire drawing, and the same number is a different thickness in every material.

One gauge number therefore covers a spread of about 0.3 mm depending on what the sheet is made of, which is larger than the whole ISO 2768 medium band at small sizes. Always convert from the stated thickness, never from the gauge number alone. Plate, once it gets thick enough to be called plate, is quoted in fractions instead and the conversion is clean: 1/8 in is 3.18 mm, 1/4 in is 6.35 mm, 3/8 in is 9.53 mm, 1/2 in is 12.70 mm, 3/4 in is 19.05 mm and 1 in is 25.40 mm.

Reading millimetres on a drawing

On an ISO style engineering drawing the millimetre is the default unit and the symbol is usually omitted, so a bare number in a dimension is already millimetres. If you see a unit written out, that is the exception being flagged, not the rule being confirmed. Getting this backwards is the quietest way to turn a 38.5 mm feature into something no supplier can make.

The style conventions differ by system too. Under ASME Y14.5 a metric dimension takes a leading zero and drops any trailing zero, so 0.5 rather than .5, and the number of decimal places in a metric dimension does not have to match the number in its tolerance, which is a rule that does apply to inch dimensions. If you are converting a whole drawing rather than one number, convert once at full precision, apply the drawing's rounding rule at the end, and never feed an already rounded value into the next calculation. Rounding compounds, and a dimension that passes through two roundings can drift further than the tolerance it was drawn with.