5

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?

Wimateeka
  • 1,091
  • 3
  • 13
  • 25
  • As Sned at Cornell used to say: "A mil is a measure of angle." see: https://en.wikipedia.org/wiki/Milliradian#Definitions_for_maps_and_artillery – D Duck Feb 15 '22 at 09:50

3 Answers3

8

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
Wimateeka
  • 1,091
  • 3
  • 13
  • 25
  • 5
    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. – Rory Alsop Feb 14 '22 at 17:24
  • 4
    Mil ( 0.001 in.) very commonly used world wide when discussing corrosion amount or rates. Also thickness of protective coatings ( paint). – blacksmith37 Feb 14 '22 at 19:39
  • 2
    @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 – Chris H Feb 14 '22 at 21:02
  • 2
    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 Feb 15 '22 at 05:51
  • @chris yet mil predates English usage in other languages like Spanish. – cde Feb 15 '22 at 08:17
  • @cde it's straight from Latin so it's not really surprising; it's also the logical choice in metric /SI prefixes. I would put the onus on the people using archaic units to deal with the confusion. Here in the UK we use both metric and imperial, but our modern engineering is firmly metric – Chris H Feb 15 '22 at 08:42
  • @chris so you are used to millimeters right? A thousands of a meter. Mil predates Thou in English usage by almost 100 years. And English the klepto it is imported thou from Germanic while most SI units are latin in nature. – cde Feb 15 '22 at 09:08
  • @cde Yes, I'm used to mm, and nearly all my measuring kit is metric-only. Whitworth was apparently using "thou" before 1850 (though I can only find reference to "thousandths" in sources I have access to), and was one of the first to propose working in decimal inches. He would have had the choice of Latinate or Germanic vocabulary to derive his terminology. "mil" from milliradian has several similar angular meanings as well. – Chris H Feb 15 '22 at 10:16
  • It bugged me that the first table gives 10 mil, which is 10/1000th or 1/100th of an inch, equal to 1/64th of an inch, which is 0.015625 inches, or about 15 mils. But it's not a transcription error, so I have to wonder if it's an error in the source or whether it's really conventional wisdom in the USC system. As a side note for @cde, most SI prefixes derive from Greek, with a handful from Latin, French, Spanish or Danish. – AmiralPatate Feb 15 '22 at 10:31
  • @AmiralPatate it's an error in the table, I reckon a flawed attempt to give some approximate conversions. All the fractional values are off - obviously as sums of powers of 1/2 can't represent powers of 1/10 precisely - but some by more than others. The one you spotted is too far off to be a convenient approximation given that 15 mil (0.381mm) is listed and 1/64" is 0.397mm. I do just enough with US-built kit to have a table of decimal inches, fractional inches, and mm on the wall next to my desk here in work. – Chris H Feb 15 '22 at 13:44
  • Also The term "mil" is not an abbreviation but a unit of measure from the source is wrong. At best they could say "no longer an abbreviation" but the full form "milli-inches" is often seen in old documents – Chris H Feb 15 '22 at 13:46
  • 3 mil is the thickness of a heavy-duty "contractor" trash bag, not regular white kitchen bags. In case you want another couple reference points, I have 0.31 mil plastic sheeting to cover windows while painting; it's light and crinkly, flimsier than a grocery sack. 6 mil is one of the heaviest I've commonly found in big-box hardware stores (I used it to seal off lead abatement work). – MissMonicaE Feb 15 '22 at 14:49
  • 2
    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 Feb 15 '22 at 16:26
1

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.

cde
  • 111
  • 3
0

"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.

Mark Morgan Lloyd
  • 131
  • 1
  • 2
    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 Feb 15 '22 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. – Mark Morgan Lloyd Feb 16 '22 at 11:12