What a mil is, and why it isn't a millimeter
The Mils to mm Converter turns a thickness written in mils into millimeters. Type 6, read 0.1524 mm. It runs in the tab you already have open, there's no account, and the answer updates as you type. The reason this page exists at all is the four-letter word in the middle: a mil is not a millimeter, and reading it as one is an error of about 39×.
The mil (thou)
A mil is one thousandth of an inch. British and Commonwealth machinists call the same unit a thou, and the two words mean exactly the same measurement — 5 thou and 5 mil are both 0.127 mm. It is the unit the United States uses whenever something is too thin to describe in fractions of an inch: plastic sheeting, trash-can liners, shrink wrap, vinyl, paint film, the copper on a circuit board. Nobody wants to write "0.006 inch" on a product label forty times, so the trade dropped the decimal and said 6 mil.
The millimeter
A millimeter is one thousandth of a meter, and it is what almost every spec sheet outside the US wants. One millimeter is 39.37 mil. Put another way: a mil is a very small unit and a millimeter is a merely small one, and the gap between them is where the money goes.
Here's the mistake in practice. Someone quotes you 6 mil poly for a ground vapor barrier. You order 6 mm. What arrives is a sheet of plastic thicker than a credit card, roughly forty times the material, at many times the price — and too stiff to do the job anyway. The Mils to mm Converter is built to stop that specific error: type a millimeter, micron, inch, or film-gauge figure into the box and it refuses, then hands you the mil figure you should have typed instead of quietly converting the wrong quantity.
How mils to mm conversion works
Multiply by 0.0254. That's the whole calculation.
mm = mils × 0.0254 — because a mil is one thousandth of an inch (÷ 1,000) and an inch is exactly 25.4 mm (× 25.4). 25.4 ÷ 1,000 = 0.0254.
Worked through with the number people look up most: 6 mil × 0.0254 = 0.1524 mm. The same thickness is 152.4 microns, and 0.006 inch. Those three readings all come back together on the Mils to mm Converter's result card, because film and coating specs outside the US are usually written in microns while the mil itself is an inch measurement — whichever one the form in front of you wants, it's on screen.
If you'd rather do it in two easier steps, divide by 1,000 to get inches, then multiply by 25.4. And for a sanity check you can run in your head, divide mils by 40: 200 mil is roughly 5 mm, and the exact answer is 5.08 mm.
The constant is exact, not measured. The 1959 International Yard and Pound Agreement fixed the international inch at exactly 25.4 mm, replacing several slightly different national inches that had drifted apart. Since a mil is defined as one thousandth of that inch, 1 mil = 0.0254 mm with no approximation anywhere in the chain. Any rounding you ever see on this conversion comes from a display deciding how many decimals to show — which is why this one shows six, so the defining 0.0254 stays visible instead of collapsing to 0.025.
Common mils to mm conversions
These are the thicknesses that actually get looked up, with the microns column for anyone filling in a metric spec.
| Mils | Millimeters | Microns | Where you see it |
|---|---|---|---|
| 0.5 mil | 0.0127 mm | 12.7 µm | Thin shrink and stretch film |
| 1 mil | 0.0254 mm | 25.4 µm | One thousandth of an inch — the definition |
| 1.4 mil | 0.03556 mm | 35.56 µm | About one ounce of PCB copper |
| 2 mil | 0.0508 mm | 50.8 µm | Light-duty bags, tape backing |
| 3 mil | 0.0762 mm | 76.2 µm | Laminating pouches, window film |
| 4 mil | 0.1016 mm | 101.6 µm | Contractor trash bags |
| 6 mil | 0.1524 mm | 152.4 µm | Poly sheeting, ground vapor barrier |
| 10 mil | 0.254 mm | 254 µm | Heavy-duty liner — exactly a quarter millimeter |
| 20 mil | 0.508 mm | 508 µm | Pond and pool liner |
| 30 mil | 0.762 mm | 762 µm | Roofing membrane, heavy geomembrane |
| 60 mil | 1.524 mm | 1,524 µm | Thick single-ply roofing |
| 100 mil | 2.54 mm | 2,540 µm | A tenth of an inch, to the digit |
Two anchors are worth memorizing because they turn up constantly. 10 mil is exactly 0.254 mm — a hair over a quarter of a millimeter — so anything in the 10-to-40 mil range is roughly its mil figure divided by 40, in millimeters. And 1,000 mil is exactly 25.4 mm, which is just the inch wearing a different hat.
Going the other way is the same constant upside down: 1 mm is 39.37 mil, so a 100 micron film is about 3.9 mil and a 12 micron shrink wrap is 0.47 mil. If a supplier in Europe quotes microns and your spec is in mils, that's the number you're comparing against.
Sub-mil values read exactly too. Half a mil is 0.0127 mm, and the converter will take .5 without a leading zero.
When you'll need to convert mils to mm
Almost always because two documents disagree about which country they're from.
Buying plastic sheeting. The roll is labelled 6 mil, the drawing or the building-inspection form wants millimeters. Same for pond liners, crawlspace encapsulation, and concrete curing blankets.
Ordering film from overseas. A US spec says 3 mil; the quote comes back in microns. 3 mil is 76.2 µm, so a 75 µm offer is a hair thin and an 80 µm offer is a hair thick — and now you can say which by how much.
PCB design. Board houses in the US quote copper weight and trace geometry in mils; the CAD file and the fab drawing are usually metric. One ounce of copper is about 1.4 mil, which is 0.03556 mm.
Paint and powder coating. Dry film thickness gets specified in mils on American job sites and in microns nearly everywhere else. A gauge reading of 4 mil is 101.6 µm.
Reading an older drawing. If it says "thou," it means mil. Machinists in the UK, Australia, and Canada write it that way, and seeing both words on one drawing doesn't mean two different tolerances.
Edge cases and gotchas
The unit traps on this conversion outnumber the math by a wide margin, so the Mils to mm Converter catches them out loud rather than converting something you didn't ask for.
| You type | What comes back |
|---|---|
6 mm | "That's already in millimeters. 6 mm is already the answer — that's 236.220472 mil going the other way." |
0.25 in | "That's inches, not mils — 1 in = 1,000 mil. 0.25 in is 250 mil — type that." |
600 gauge | "That's film gauge — gauge is 100× mils. 600 gauge is 6 mil — type that." |
150 microns | "That's microns, not mils — 1 mil = 25.4 µm. 150 µm is 5.905512 mil — type that." |
1,250 | Converts, with a note: "Read as 1,250. If you meant 1.25, type it with a dot." |
-3 | "Mils can't be negative. Use 0 or more." |
Gauge is not a gauge. In the plastic film trade, gauge is mils × 100 — 100 gauge is 1 mil, 600 gauge is 6 mil. It exists so thin films can be quoted as whole numbers. It has nothing to do with sheet metal or wire gauge, where a bigger number means thinner material and there's no clean multiplier at all.
"Mil" means other things elsewhere. In Norway and Sweden a mil is 10 kilometers. In shooting and artillery a mil is an angle, roughly a thousandth of a radian. In casual European writing it sometimes stands in for millimeter. This page handles only the thickness mil — thousandths of an inch — which is the one you want whenever the number describes how thick a sheet, film, coating, or copper layer is.
Comma placement. 1,250 is read the American way, as one thousand two hundred and fifty. If you write decimals with a comma, use a dot here instead.
Related converters
If your figure is in inches rather than thousandths of one, the inches to mm converter takes it directly — 0.25 in and 250 mil are the same thickness, and either page gets you to 6.35 mm. Coming back the other way, mm to inches handles drill bits, jewelry, and screen sizes. For thicknesses that have grown past the millimeter, cm to mm is a decimal shift you can do in your head but shouldn't have to at 7am on a job site.
When you're working through a whole spec sheet rather than one number, the length converter covers every metric and imperial pairing in one place, and metric to imperial handles the mixed-unit documents that make this conversion necessary in the first place.
All of them work the same way this one does: open, convert, leave. Nothing to install, no account, no counter ticking down toward a plan. A thickness chart is not a product — it's a fact, and facts shouldn't sit behind a supplier's contact form or three clicks deep inside a platform billing you monthly per seat.
Frequently asked questions
How many mils are in a millimeter?
39.37, near enough — the exact figure is 39.3700787 mil. It's just 1 ÷ 0.0254. So a 100 micron film is 3.94 mil, a 12 micron shrink wrap is 0.47 mil, and a 1 mm sheet is a shade under 40 mil. If you need the reverse direction often, the ÷ 40 shortcut works in your head and lands within 2%.
My drawing says 0.006 inch. Is that 6 mil?
Yes. A mil is a thousandth of an inch, so multiply inches by 1,000 to get mils: 0.006 in = 6 mil = 0.1524 mm. Type 0.25 in into the box and it'll tell you the same thing — that it's 250 mil, and to type that instead. Drawings switch between the two notations freely, sometimes on the same sheet.
What is 600 gauge in millimeters?
0.1524 mm. Gauge in the film trade is mils × 100, so 600 gauge is 6 mil, which is the same 0.1524 mm as the poly sheeting above. 400 gauge is 4 mil (0.1016 mm), and 100 gauge is 1 mil (0.0254 mm). If a quote is in gauge and the drawing is in millimeters, divide by 100 first.
Why does the result show six decimal places?
Because three would be a lie. The defining value, 1 mil = 0.0254 mm, rounds to 0.025 at three decimals — and the whole point of this conversion is that it's exact rather than approximate. Six decimals keeps every common thickness intact, including awkward ones like 1.4 mil = 0.03556 mm. The number above a million switches to comma grouping instead, so nothing ever comes back in scientific notation.
Can I type "6 mil" or "6 thou" instead of just the number?
Both work. The Mils to mm Converter also takes 6 mils, 6 MILS, and .5 without the leading zero. Anything ending in mm, µm, in, or gauge gets stopped with the mil figure to type instead. A trailing unit that means mils is read as mils; a trailing unit that means anything else is treated as the mistake it probably is.
Is any of this sent anywhere, and what does it cost?
Nothing is sent and nothing is stored — the conversion happens in your browser, and closing the tab clears it. The converter costs nothing to use. Microapp gives 10% of every dollar it earns to charity, off the top and audited quarterly, which is a better use of a thickness chart than making you fill in a lead form to see one.