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?
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)
Additionally, the same website had some helpful information for visualizing thickness in mils:
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 10 mil Business card 6 mil white trash bag used in kitchens 4 mil standard piece of paper
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.
