What does 'mil' mean when measuring thickness?

I asked a question about transparencies, and while doing research for it, I found a lot of websites mentioned their products were N mil thickness.

What does mil mean when talking about the thickness of a material? How does it compare to other common units of measure?

• As Sned at Cornell used to say: "A mil is a measure of angle." see: en.wikipedia.org/wiki/… Commented Feb 15, 2022 at 9:50

The internet has informed me one mil is a measurement that equals one-thousandth of an inch (0.001 inch) or 0.0254 mm. You can convert any length unit into mils via Google's length unit conversion (Google labels it Thousandth of an inch instead of mil).

Per the comments below, there are some British English applications where the term thou (short for thousands) is used instead of mil.

I also learned that to convert between mil and gauge one can take the number of mil and multiply by 100; thus a 2 mil thickness converts to 200 gauge (according to ULine's website).

Explanation of mil I found from a helpful plastic sheeting website:

How does mil thickness compare to mm (millimeter) and inches? What does mil stand for in measurement?

Plastic sheeting is measured in mils. A mil is a measurement that equals one-thousandth of an inch, or 0.001 inch. One mil also equals 0.0254 mm (millimeter). Thus a mil is not the same thickness as a millimeter. The term "mil" is not an abbreviation but a unit of measure.The chart below gives you an idea of mils to millimeters to inches. An every day trash bag ranges between 1.2 mils and 1.7 mils. A much stronger trash bag that offers better tear resistance is between 3 mils and 6 mils. A credit card is around 30 mils thick while a common deck of playing cards including the box is approximately .75 inches thick or 750 mils.

mil mm inch Item
1 0.0254 0.001
3 0.07619 0.003
6 0.152399 0.006
10 0.254 0.01 (1/64 in)
15 0.381 0.015
20 0.508 0.02
30 0.762 0.03 (1/32 in) Credit card
60 1.524 0.06 (1/16 in)
100 2.54 0.1 (3/32 in)

Thickness of Plastic Sheeting

Plastic Sheeting [...] is rated with the term, "mil". [...] Most human hair is One-thousandth of inch, or 0.001 inch [or 1 mil]. The most common size in the thickness rating for plastic sheeting is 6 mil. This is 6-thousandths of an inch, or 0.006 inch. Generally, the thicker the plastic, the stronger it is. If it has string/scrim reinforcement, then the string within the plastic will give it added strength. Depending on the application, the composition of the plastic can play a big role regardless of the thickness. For example, additives added to the plastic such as the fire retardant (FR) additive may play a more significant role than the thickness of the plastic if the job requires FR plastic.

Note that the term " heavy gauge" plastic is not to be confused with "heavy mil" referring to heavy plastic sheeting. The term "gauge" refers to the thickness of metal. It really doesn't apply to plastic sheeting/ plastic sheets.

Thickness of Plastic Sheeting Common item for comparison
75 mil U.S. Nickel
60 mil U.S. Penny
50 mil U.S. Dime
6 mil white trash bag used in kitchens
4 mil standard piece of paper
• That is definitely news to me. In non-Imperial measurement countries, such as the one I live in, I have always used mil as millimetre, because we only use metric measurements for this sort of thing. Commented Feb 14, 2022 at 17:24
• Mil ( 0.001 in.) very commonly used world wide when discussing corrosion amount or rates. Also thickness of protective coatings ( paint). Commented Feb 14, 2022 at 19:39
• @RoryAlsop hence 0.001" being called "thou" in British English. "Mil" as used here is essentially an Americanism that has gone international in some uses Commented Feb 14, 2022 at 21:02
• I totally agree with Rory Alsop; I thought mil was a different abbreviation for millimetre. Kind of makes sense though... If you base your unit of measurement on the length of 3 corns of barley and you want to derive a much smaller unit of measurement you "mill" them down XD
– Elmy
Commented Feb 15, 2022 at 5:51
• Mil are never used as short version for milimeters in my experience (in speech, yes, written, never), but you'll see it a lot in printed circuit board (PCB) design. Many measurements on a PCB are in mil.
– Mast
Commented Feb 15, 2022 at 16:26

This is basic English etymology:

mil (n.)

1721, in per mil "per thousand," from Latin mille "thousand" (see million); compare percent. As a unit of length for diameter of wire (equal to .001 of an inch) it is attested from 1891; as a unit of angular measure it is recorded by 1907. https://www.etymonline.com/word/mil

Like a large part of English and typical imperial units, mil comes from Latin for thousands. As in One Thousands of an inch. Mil is also the root word for million. And milli as in milliamps or millimeters, in metric units. Mil is a the root of them all.

This is also true for other languages like Spanish, where a mil pesos for example means a thousand as in One Thousand Dollars.

"mil" is not a standardised unit, and whilst it can usually be interpreted as "one thousandth" of something the only safe course of action is to ask the supplier for clarification.

Having said that, it's probably safe to interpret it by context (e.g. paper would be unlikely to be several millimetres thick) and by reference to the location of the seller (Americans use English units, just about everybody else uses metric units). But you'd be most unwise to rely on that if gross misjudgement would be expensive: a court would probably not consider "I assumed that..." to be an adequate level of certainty.

• This answer is wrong. In certain industries, mil is used as a standardised unit. Perhaps not SI standardised, but in those sectors (like electronics R&D) it's very standard. 0.001".
– Mast
Commented Feb 15, 2022 at 16:28
• @Mast There are certain environments in which "Furlongs per Fortnight" and measuring lengths by comparing them with that of a London bus are popularly accepted. Quoting a length in mil has no more standing than that. Commented Feb 16, 2022 at 11:12