Convert in to mm

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.

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.

The factor is 25.4, and almost nobody carries that around. Rounded to 25 it is off by 1.6 % — which stays invisible on small numbers and turns into a whole unit somewhere around 100 in.

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 figure 25.4 was chosen rather than found. Industry had been drifting towards it for decades before the governments caught up — an inch that is a clean 25.4 mm makes every gauge, every thread table and every conversion chart agree — and in 1959 six countries stopped maintaining their own slightly different inches and adopted it outright. The foot, the yard and the mile were all redefined in the same document as multiples of it.

The practical consequence for a workshop is that inch and metric dimensions are the same system with two notations. A drawing in inches can be converted to millimetres and back without drift, and a disagreement between two converters is always a rounding choice. Where two metric figures for the same inch part differ, one of them was rounded before the multiplication rather than after.

Precision work in inches is counted in thousandths, and the conversion is worth memorising: one thou is 0.0254 mm, or 25.4 µm. Five thou is 0.127 mm, ten thou is 0.254 mm, forty thou is 1.016 mm. Feeler gauges, shim stock and surface finishes all arrive in these units, and a metric machine wants them in millimetres to two or three decimals.

The rough equivalence is that a thou is 25 microns and a bit — the bit being four tenths of a micron — so the two are the same thing for any purpose where you were going to say "about". Where you were not going to say "about" — a bearing fit, a shim stack, a press interference — carry the exact figure, because five thou called 0.13 mm instead of 0.127 mm is more than two per cent of error on a clearance that was specified to a per cent.

The sizes an American workshop actually uses are binary fractions, and their millimetre equivalents terminate quickly because 25.4 has only one decimal place to give away. A sixty-fourth is 0.396875 mm, a thirty-second 0.79375, a sixteenth 1.5875, an eighth 3.175, a quarter 6.35, three eighths 9.525, a half 12.7, three quarters 19.05. Every one of those is exact.

What they are not is close to any metric stock size. A quarter-inch bar is 6.35 mm and metric bar comes in 6 mm; half-inch plate is 12.7 mm against 12 mm; three-quarter tube is 19.05 mm against 20 mm. There is no fractional inch size between 1/8 and 1 inch that coincides with a preferred metric size, which is why substituting one for the other is always a design change and never a swap.

This is the failure that costs a job. A 1/4-20 UNC screw has a major diameter of 6.35 mm and twenty threads to the inch, which is a 1.27 mm pitch; an M6 has a 6.00 mm major diameter and a 1.0 mm pitch. The head sizes differ too, so the spanner that fits one rounds the other. Nothing about the pair is interchangeable, and the fact that both are described as "six millimetre-ish" is the whole of the trap.

The damage is usually done by hand. An M6 bolt will start in a 1/4-20 nut for a turn and a half before it binds, and the force that gets it past that point strips the softer of the two threads. The check is the pitch rather than the diameter: count the crests over ten millimetres, or put a thread gauge on it, before anything is tightened.

Plenty of inch measurements survive inside otherwise metric industries because the interface was standardised first and the cost of changing it would be borne by everybody. Server racks are 19 inches, which is 482.6 mm, and a rack unit is 1.75 inches, or 44.45 mm — dimensions printed in millimetres on European datasheets and derived from inches every time. Vinyl records are 12 and 7 inches, or 304.8 and 177.8 mm.

Bicycles are a mixture that catches people out: wheel sizes are quoted in inches, a common steerer tube is 1 1/8 inch or 28.575 mm, and the bottom bracket may be threaded 1.37 inch by 24 threads to the inch while the frame around it is drawn entirely in millimetres. Where a component is described in inches, convert it and then check the number against the standard, because the inch figure is often a name rather than a size.

A dimension without its tolerance is half the information, and the tolerance converts with the same factor. Plus or minus five thou is ±0.127 mm; ±0.001 inch is ±0.0254 mm; ±1/64 inch is ±0.396875 mm. The trap is rounding the tolerance to something tidy in millimetres, because rounding it outward loosens the part and rounding it inward makes it harder to produce than the designer intended.

The convention that avoids arguments is to carry one more decimal place in the millimetre tolerance than the inch one implied, then round the limits rather than the tolerance itself — compute the upper and lower size in millimetres and round each to the same number of decimals. That keeps the converted zone inside the original one, which is the safe direction to fail in when the original drawing is the controlling document.

Metric engineering drawings are dimensioned in millimetres and by convention the unit is not written next to the numbers — a note in the title block says the drawing is in millimetres and every figure is understood. That convention exists because it removes a unit from every dimension on a crowded drawing, and it works exactly as long as nobody introduces a second unit.

A centimetre figure on such a drawing is not read as a centimetre figure; it is read as a millimetre figure and cut ten times too small. This is the reason to convert inches to millimetres directly rather than through centimetres, and the reason a converted inch dimension should be written with its decimals rather than tidied: 6.35 is unmistakably a millimetre figure where 0.635 invites a question nobody will stop to ask.