How this instrument works
Mil borrows its name from mille, and it labels the inch's thousandth part by definition rather than by measurement. That makes 0.001 an exact factor carrying no uncertainty whatsoever: converting is decimal bookkeeping, nothing more. British and Australian workshops say thou instead, precisely to stop this quantity being misheard as millimetre. Both words describe the same span — 25.4 micrometres, once you fix an inch at 25.4 mm.
Unlike most conversions, this one never required an international treaty. Because the mil sits inside the inch system as its thousandth part, the ratio holds whichever inch you invoke, and only crossing into metric makes the size of that inch matter. What thousandths did require was instruments able to see them: Brown & Sharpe put pocket micrometer calipers graduated straight in 0.001 in on the American market from 1867, and once workmen could read that increment, drawings began carrying decimal inches instead of halves and sixteenths.
Circuit designers still lay out on thousandth-inch grids: 0.1 in DIP pitch is 100 mil, ordinary board stock measures 62 mil, and 8 mil traces with 8 mil clearance suit budget fabrication. Wire gauge speaks that language too — AWG 30 runs almost exactly 10 mil across — and North American cable tables scale it into circular mils, an area unit sold by the thousand as kcmil. Anodisers, platers and shim-stock suppliers all quote thousandths as well.
- Type your thickness into Mils (thou) — 250 sits there as a starting point.
- Read Inches (in) directly beneath; it recomputes on every keystroke.
- Sanity-check by eye: shift one decimal point three places left and you should land on that same figure.
- Working backwards from inches? Multiply by 1000 — 0.032 in of shim stock is 32 mil.
- Negative entries are refused, since thickness and gauge start at zero.
Worked example — 250 mil gasket sheet
A drawing calls for nitrile gasket material 250 mil thick, but every rack on your stock wall is labelled in fractions. Enter 250 into Mils (thou) and Inches (in) returns 0.25 — one flat quarter inch, so quarter-inch sheet is what you cut. Metric equivalent, should the supplier ask: 6.35 mm.
That shift works just as cleanly downward. Brass shim at 7.5 mil is 0.0075 in on the micrometer, and 62 mil board stock is 0.062 in, which explains why everyone calls that thickness sixteenth-inch even though true sixteenths run 62.5 mil. Shop speech goes finer still: one tenth means 0.0001 in, one tenth of one single thou.
Questions
Is a mil the same as a millimetre?
No, and confusing them produces roughly fortyfold error. One mil is 0.0254 mm, so 62 mil board stock measures 1.57 mm, not 62 mm. That risk is why thou remains standard usage across British and Australian shops. Two other meanings lurk nearby: across Sweden and Norway one mil means 10 kilometres, while riflemen mean an angle by it — the milliradian, roughly 3.6 inches at 100 yards.
Is 0.001 exact, or has it been rounded?
Exact. The mil is defined as the inch's thousandth part, which makes this arithmetic rather than measurement, so no uncertainty enters anywhere. Multiplying by 0.001 moves one decimal point three places left; reversing multiplies by 1000. Every digit returned here is exactly as trustworthy as whatever digits you typed.
Does imperial versus US customary change my answer?
No — and this pairing is unusual in that the distinction cancels outright. Because a mil is the thousandth part of whichever inch you are working in, the ratio stays 1:1000 for the international inch, for its pre-1959 British ancestor, and for the US survey inch that NIST retired at the end of 2022. Those inches disagree by around two parts per million, but that disagreement lives in the inch itself. It surfaces only once you carry either figure across into millimetres or microns.
What is a circular mil, and can I convert it here?
It measures area, not length, so no. One circular mil is the area of a circle 1 mil in diameter, equal to π⁄4 ≈ 0.7854 square mils. North American conductor tables run on this unit: one thousand circular mils make one kcmil, and 250 kcmil feeder cable works out slightly over half an inch in diameter. Feed kcmil figures through any length converter and you will get nonsense.
How do fractional inch sizes translate into mils?
Cleanly, because binary fractions terminate in decimal. One sixteenth works out at 62.5 mil, one thirty-second at 31.25, one sixty-fourth at 15.625, and 3⁄8 in at 375. Multiply any inch figure by 1000, or slide one decimal point three places right. Shop stock follows suit: 0.125 in acrylic is 125 mil, 0.090 in styrene is 90 mil.
What everyday things measure about one mil?
Kitchen foil sits near 0.6 mil and cling film thinner still. One sheet of 20 lb office paper comes close to 4 mil, human hair spans 2 to 4 mil, AWG 30 magnet wire is 10 mil across, and an American dime stands 53 mil tall. Instruments built for this work resolve to one tenth of one thousandth, which is what shop talk calls a tenth.