How this instrument works
Microns are millionths of a metre. The SI name is micrometre, symbol µm, and one of them spans roughly a seventieth of a human hair's width. CGPM sanctioned micron as an official name in 1879, then struck it from SI vocabulary at its 13th meeting in 1967, together with bare Greek mu used as a symbol. Industry ignored that. Vacuum technicians, filter makers, semiconductor fabs and film extruders still say micron, and mean precisely what metrologists mean by micrometre.
Mils are thousandths of an inch. British and Commonwealth machinists call it thou, specifically to keep it clear of the millimetre, but both names describe one identical length. Since an inch has been exactly 25.4 mm under the 1959 international yard and pound agreement — which American and British manufacturers had already adopted informally during the 1930s — one mil is exactly 25.4 µm. Reverse that and one micron becomes 5⁄127 mil, an ordinary rational number whose decimal expansion, 0.03937007874015748…, repeats forever yet carries no uncertainty whatsoever.
Both units survive because two measuring cultures coat and laminate identical goods. An American paint inspector reports dry film thickness in mils; the ISO datasheet for that same coating specifies microns. Builders buy 6-mil polyethylene vapour barrier, which European merchants list as 150 micron sheeting. Board shops quote copper foil by weight — one ounce, meaning about 1.37 mil or 35 µm — and draw trace widths in mils while their fab abroad works entirely in microns. Each of those handoffs is this one multiplication.
- Type your thickness into the Microns (um) field; 100 um is preloaded as your starting point.
- Read the answer on the Mils (thou) (mil) line — it recomputes as you type, with no button to press.
- Round only at the end, and only to digits your gauge can genuinely resolve, which for most film thickness meters means two decimal places.
- Working backwards from mils? Multiply by 25.4 instead; that factor is exact and needs no calculator memory.
Worked example — 100-micron powder coat, read in mils
The European datasheet calls for 100 µm of dry film on your powder-coated bracket. Your inspector arrives holding one magnetic gauge that reads only in mils. Enter 100 into Microns (um) and read 3.93700787402 on the Mils (thou) (mil) line, then write 3.94 mil on the report, because no handheld gauge resolves beyond two decimals.
That figure also settles one argument worth having. Shop specifications written as 4 mil correspond to 101.6 µm, so your 100 µm coat runs about 1.6% thin of that — inside the typical ±10% band, though worth flagging if adhesion is marginal. Scaling stays trivial in either direction: 25.4 µm is one mil precisely, your 2-mil primer coat is 50.8 µm, and 12 µm household foil measures just under half of one mil.
Questions
Is one mil exactly 25.4 microns?
Yes, exactly — by definition rather than measurement. One mil is one thousandth of an inch, and an inch has been fixed at 25.4 mm since six standards bodies signed the 1959 international yard and pound agreement. Multiply through and 0.001 inch becomes 25.4 µm with zero uncertainty. Going from microns to mils divides by 25.4, giving 5⁄127, whose decimal never terminates; the 0.0393700787402 shown here is that fraction rounded at twelve significant figures.
Is one mil the same as one millimetre?
No, and confusing them is the classic expensive error. One mil is 0.0254 mm — roughly forty of them fit inside one millimetre, since 1 mm is 39.37 mil. Two further traps share that spelling: in Sweden and Norway, mil colloquially means 10 kilometres, and in gunnery and rifle optics an angular mil is 1⁄6400 of one full turn. Only the thousandth-of-an-inch meaning applies to sheet, film, and coating thickness.
Why do British engineers say thou rather than mil?
To avoid exactly that millimetre collision. British and Commonwealth workshops adopted thou, short for thousandth, so spoken figures could never be mistaken for metric ones across noisy shop floors. Thou and mil are numerically identical, and older drawings sometimes write it as 0.001 inch or with double-prime decimals instead. Whichever label appears on your print, feed the number through this same 25.4 conversion.
Does film gauge mean the same as mils?
No. In American plastics, gauge means one hundredth of one mil, so 100 gauge equals 1 mil equals 25.4 µm. An 80-gauge stretch wrap is therefore 0.8 mil, or about 20.3 µm; the 400-gauge liner is 4 mil, near 101.6 µm. Divide gauge by 100 before converting, otherwise your answer lands one hundredfold out. Paper caliper carries its own trap: printers call one mil one point, unrelated to the typographic point.
Is the micron still an official unit?
Not officially. The 13th CGPM removed micron from the SI in 1967, leaving micrometre and the symbol µm as correct usage, and NIST Special Publication 811 discourages micron in technical writing. Nothing changed numerically, though — micron and micrometre denote an identical length. Filtration ratings, vacuum readings, anodising specs, and extrusion sheets carry on using micron, and this converter treats both words as one unit.
Do imperial and US customary inches give different answers?
Not for this conversion. Since 1959 both systems use an identical 25.4 mm inch, so one mil is 25.4 µm on either side of the Atlantic. One historical exception never touched thin-film work: the US survey foot, retired at the end of 2022, implied survey inches about two parts per million longer. That discrepancy shows up over kilometres of land survey baseline, never across coatings just microns thick.