How this instrument works
An inch is 25.4 millimetres — not thereabouts, but exactly, by law. Britain's engineering standards committee adopted that figure for industry around 1930, American manufacturers followed soon after, and on 1 July 1959 six national standards bodies made it common across their countries through the international yard and pound agreement. Before then, American inches measured 25.4000508 mm, inherited from Mendenhall's 1893 order pegging the yard to 3600⁄3937 of one metre, while British inches ran fractionally short at roughly 25.399956 mm.
Travelling toward inches means dividing by 25.4, which is where tidy decimals give out. 1 ÷ 25.4 = 0.0393700787401574803… forever, because 25.4 hides the prime factor 127, and 127 will never divide any power of ten. This sheet multiplies instead by 0.0393700787402 — that same quotient carried to twelve significant figures. Across one full metre of bar stock, what you surrender amounts to roughly one picometre, well under the width of any single atom, far below whatever your machine tools or gauge blocks could ever resolve.
Millimetres run drawing offices worldwide: ISO mechanical drawings dimension everything as millimetres by default, often printing no unit symbol whatsoever. Lens focal lengths, spanner sizes, bicycle frames, cartridge calibres, circuit-board pads all arrive that way. Inches hold their ground across North American construction, plumbing schedules, screen diagonals and wheel rims — even Tokyo-built cars still roll on 17-inch wheels. Most engineers therefore read both scales daily, and the machinist's thou, one thousandth of an inch or precisely 25.4 micrometres, sits as the hinge between them.
- Type your measurement into the Millimetres (mm) field — 25.4 arrives preloaded, since that value defines an inch.
- Read your answer off the Inches (in) output line; it recomputes with every keystroke.
- The field steps by 0.001 mm, so micron-level dimensions lifted straight off a drawing paste through without loss.
- Need a shop fraction? Multiply those decimal inches by 64 and round to the nearest whole number — that numerator over 64 gives your nearest sixty-fourth.
- Travelling backwards, inches to millimetres, multiply by 25.4 instead. That direction is exact and easy enough to do standing at a bench.
Worked example — 25.4 mm on a supplier drawing
A German supplier's drawing calls a shoulder diameter out at 25.4 mm. Type 25.4 into Millimetres (mm) and Inches (in) returns 1.0 — dead on one inch, because 25.4 mm is what an inch was defined to be. That suspiciously round result is a tell: whoever drew that part almost certainly worked from an inch-dimensioned original and converted.
Run further items down that same parts list and a pattern keeps surfacing. 12.7 mm reads 0.5 in, 6.35 mm reads 0.25 in, 3.175 mm reads 0.125 in — a chain of halvings from 25.4. Genuinely metric features land somewhere untidy by comparison: 6 mm gives 0.2362 in, which matches no drill on any imperial rack.
Questions
Is 25.4 mm per inch exact, or is it rounded?
Exact, and exact by definition rather than by measurement. Six standards bodies — American, British, Canadian, Australian, New Zealand and South African — fixed a yard at 0.9144 m from 1 July 1959, which makes an inch 25.4 mm with zero uncertainty attached. What does get rounded is travel toward inches: 1 ÷ 25.4 has no terminating decimal, so this page carries 0.0393700787402, twelve significant figures of it.
Does an American inch differ from a British imperial inch?
Not since 1959 — for length, both are identical, along with feet, yards and miles. Divergence between those systems survives elsewhere: a US gallon holds 3.785 L against an imperial 4.546 L, and their fluid ounces, pints and bushels disagree too. One length exception lingered, though. American surveyors kept a slightly longer foot of 1200⁄3937 m, about two parts per million over, until NIST and NOAA retired it at the close of 2022.
What is a thou, and how big is it in millimetres?
One thou (also called one mil) is one thousandth of an inch, or 0.0254 mm — 25.4 micrometres precisely. Toolmakers speak almost entirely this way: shim stock, feeler gauges, guitar strings and machining allowances all come quoted as thousandths. Drop another decade and you reach the 'tenth', meaning 0.0001 in or 2.54 µm, which is roughly where surface finish and thermal growth begin dominating whatever your micrometer says.
Why does my tape measure show sixteenths rather than decimals?
Because customary practice halves rather than divides by ten, giving halves, quarters, eighths, sixteenths and beyond. Decimal output therefore needs a second step for shop use. Take 3.5 mm: this page gives 0.13780 in, which falls between 1⁄8 (0.125) and 5⁄32 (0.15625), and the nearest sixty-fourth is 9⁄64 at 0.140625. Multiplying by 64 and rounding, as described above, gets you there without hunting along a chart.
Why doesn't a half-inch pipe measure 12.7 mm?
Because plumbing and lumber sizes are nominal labels, not dimensions. Half-inch NPS pipe runs 21.3 mm outside diameter regardless of wall thickness, following conventions inherited from old iron-pipe bore tables. American 2×4 lumber leaves the mill at 38 × 89 mm once dried and planed. Converting either label yields numbers that measure nothing real, so reach for your manufacturer's dimension table instead of this or any other converter.
How many decimal places should I keep for machining?
Match whatever your tolerance allows, then one spare digit. General machine-shop work holds ±0.1 mm, near enough ±0.004 in, so three decimal places of inches carries everything meaningful. Grinding and gauge work at ±0.005 mm needs four. Rounding earlier than that is where real error creeps in — round once at the end, never partway through a chain of features, or those small shortfalls accumulate along a stack.