How this instrument works
One mil equals one thousandth of an inch. British and Commonwealth workshops call it thou and treat that word as countable — take off two thou, leave five thou of clearance — while North American shops kept mil. Machinists' micrometers resolve tenths of thou, or 2.54 micrometres, finer than red blood cells. Since an inch measures 25.4 mm by definition, one thou measures 0.0254 mm, and that arithmetic carries no uncertainty at all.
Nothing about 25.4 was inevitable. Britain's imperial inch, traceable to a bronze yard bar cast in 1855, worked out near 25.399956 mm; America's, defined by Mendenhall's 1893 order as 3600⁄3937 of a metre, came to 25.4000508 mm. Firms machining parts on both sides of an ocean grew tired of disagreement inside shared drawings, so British Standards adopted round-number 25.4 in 1930 and American industry followed in 1933. Six governments made it law on 1 July 1959 with an international yard and pound agreement, and every thou since has been 0.0254 mm precisely.
Trades still counting in thousandths are mostly ones where thickness rules, not length. Board designers lay out copper on a grid marked in thousandths, where 40 wide carries roughly one amp on inner copper; one ounce of copper per square foot spreads to 1.37 of them. Polythene sheeting sells by that same measure — 6 for vapour barriers, 45 for pond liner — and painters comb wet coatings to check two or three of build. Anodising, plating, shim stock, feeler gauges and laminating pouches all speak it. Credit cards, handy for reference, run 30 thou thick.
- Enter your figure in the Mils (thou) (mil) field. It opens at 40, a spark-plug gap you will meet often.
- Read the answer on the Millimetres (mm) line, which recomputes on every keystroke.
- Fractions are welcome — steps of 0.001 are accepted, so 7.5 returns 0.1905 mm.
- Working backwards from a metric drawing? Divide millimetres by 0.0254; one millimetre comes to 39.37 thou.
- Need microns instead? Shift three places: 1.016 mm is 1016 µm, since a thou is 25.4 µm on the nose.
Worked example — a 40-thou plug gap
An American shop manual calls for a spark-plug gap of 0.040 in, and every blade in your feeler set is stamped in metric. Enter 40 into Mils (thou) (mil) and Millimetres (mm) returns 1.016. Your 1.0 mm blade slips through with 16 micrometres of daylight; a 1.05 mm blade jams. Gap correctly and no rounding argument is needed.
Nothing got approximated along that path: 40 × 0.0254 = 1.016, with decimals simply stopping there, which is why such figures are safe to letter onto drawings. A board designer meets identical arithmetic from another angle — 40-thou copper traces clear 1 mm metric footprints by 16 micrometres, tight but real.
Questions
Is a mil the same thing as a millimetre?
No, and confusing them is by far the costliest error with this unit. A millimetre is 39.37 times larger. Order 40 mm of plastic sheeting when a drawing wanted 40 mil and you have specified something 39 times too thick. Abbreviations invite trouble because both start with the same letter, so many drawing offices now write thou or 0.001 in instead, reserving mm for metric.
Is 0.0254 an exact factor or a rounded one?
Exact, by law rather than by measurement. An inch has equalled 25.4 mm precisely since the international yard and pound agreement took effect on 1 July 1959, binding standards bodies of six nations. Divide by a thousand and a thou is 0.0254 mm with zero uncertainty. Any digits you see truncated on screen were shortened purely for display; underlying arithmetic terminates naturally, because 254 divided by 10000 is a finite decimal.
How many microns are in a mil?
25.4 micrometres, exactly — a useful bridge, since laboratories and coating specifications often prefer microns. Multiply thousandths by 25.4 and read microns; divide microns by 25.4 to go back. Plastic film adds a third scale called gauge, where 100 gauge equals one thou, so 400 gauge film is 4 thou, 101.6 µm, or 0.1016 mm. All three describe identical stuff.
Does imperial or US customary measure change this factor?
Not since 1959 — both systems now share one inch of 25.4 mm, so a thou reads 0.0254 mm either way. Only surveying kept a separate definition: an old US survey inch of 25.4000508 mm, larger by two parts per million, which over a thousandth of an inch amounts to about 0.05 nanometres. NIST and NOAA retired that survey unit at the end of 2022 anyway, so nothing in machining or coating work is affected.
Why do British engineers say thou while Americans say mil?
Regional habit, mostly, though ambiguity gave British usage a reason to stick. Scandinavians use mil for 10 kilometres, artillery and rifle optics use it for a milliradian — 6400 to a full circle on NATO gear — and casual speech sometimes shortens millimetre to it. Thou has none of that baggage. Whichever word your drawing uses, arithmetic here is unchanged: multiply by 0.0254.
How do I convert millimetres back into thousandths?
Divide by 0.0254, which is multiplying by 39.370078740157. That number never terminates because reversing an exact definition rarely does, yet it hides an ordinary fraction, 5000⁄127, and so carries no error either. Quick anchors worth memorising: 1 mm is a shade over 39 thou, 0.1 mm sits just under 4, and 25.4 mm lands on exactly 1000.