mm to Mils Converter
A payment card is about 30 thou thick, which sounds odd until you convert it: roughly three quarters of a millimetre.
Quick answer: divide the millimetres by 0.0254, or multiply by 39.3701. So 1 mm is 39.37 mil, 0.5 mm is 19.69 mil, 2 mm is 78.7 mil and 25.4 mm is exactly 1,000 mil.
The millimeters to mils formula
A mil, also called a thou, is one thousandth of an inch, which is exactly 0.0254 mm.
Converting millimetres to mils therefore means dividing by 0.0254, or multiplying by about 39.3701.
The division is exact, because the inch is defined as exactly 25.4 mm.
For micrometres, a mil equals 25.4 µm, so you can also divide microns by 25.4.
The mil, the thou and the millimetre
A thou, or mil, is one thousandth of an inch, which equals 25.4 micrometres or 0.0254 mm. A millimetre is therefore roughly 39 mils (Wikipedia: Thousandth of an inch).
It measures thin materials such as paper, film, foil, wire, coatings and plastic sheeting, and payment cards are about 30 thou thick. Older automotive work uses it for gasket thickness and spark-plug gaps (Wikipedia: Thousandth of an inch).
Be careful with the word mil, because it can also be used informally for millimetre, which is about 39 times larger. Many American technical users therefore prefer thou.
How to convert mm to mils step by step
- Write down the length in millimetres.
- Divide it by 0.0254, or multiply by 39.3701.
- Round to one decimal for thin materials.
- Add the unit mil to the answer.
Example 1: a payment card
- Start with 0.76 mm.
- 0.76 × 39.3700787402 = 29.9213.
- Round to 29.9.
0.76 mm = 29.9 mil
Example 2: a coin thickness
- Start with 1.35 mm.
- 1.35 × 39.3700787402 = 53.1496.
- Round to 53.1.
1.35 mm = 53.1 mil
Example 3: a small bolt
- Start with 10 mm.
- 10 × 39.3700787402 = 393.7008.
- Round to 393.7.
10 mm = 393.7 mil
The card measures 29.9 mil, close to the 30 thou usually quoted, while the 10 mm bolt is almost 394 mil, or about 0.394 in.
mm to mils conversion table
| Millimetres (mm) | Mils (thou) |
|---|---|
| 0.01 mm | 0.39 mil |
| 0.025 mm | 0.98 mil |
| 0.05 mm | 1.97 mil |
| 0.1 mm | 3.94 mil |
| 0.25 mm | 9.84 mil |
| 0.5 mm | 19.69 mil |
| 0.76 mm | 29.92 mil |
| 1 mm | 39.37 mil |
| 1.35 mm | 53.15 mil |
| 2 mm | 78.74 mil |
| 2.5 mm | 98.43 mil |
| 5 mm | 196.85 mil |
| 10 mm | 393.7 mil |
| 25.4 mm | 1,000 mil |
The highlighted row is the key relationship: one millimetre is about 39.37 mil. The last row shows that 25.4 mm is exactly 1,000 mil, which is one inch.
A quick mental shortcut for millimetres to mils
Multiply by 40 and take off about 1.5 percent. For 2 mm, forty times is 80, and the small trim gives roughly 78.7 mil.
For a rougher check, remember that 0.1 mm is about 4 mil and 1 mm is about 39 mil.
Mm to mils for thin materials and coatings
Plastic sheets, polythene film, paint and powder coatings, and wire are often specified in mils. A 2 mil film is about 0.05 mm thick, and a 10 mil sheet is about 0.25 mm.
Converting millimetres to mils lets you match a caliper reading with a supplier specification.
Mil versus millimetre and the micron
Because mil is sometimes used as slang for millimetre, check which unit a drawing means. A 1 mm gap is about 39 mil, while a 1 mil gap is only 0.0254 mm.
To reach microns, multiply mils by 25.4, so 10 mil is 254 µm.
More values at a glance
0.5 millimetres is 19.685 mils. 2 millimetres is 78.7402 mils. 5 millimetres is 196.8504 mils.
10 millimetres is 393.7008 mils. 25 millimetres is 984.252 mils. 100 millimetres is 3,937.01 mils.
Each answer is the input times 39.3701. You can use the same factor for any value that is not listed here.
The unit sizes
One of the millimetres equals 39.3701 mils. One of the mils equals 0.0254 millimetres.
If a result looks very large or very small, compare it with these sizes before you use it.
Checking in reverse
Take 393.7008 mils and divide by 39.3701. You get back 10 millimetres. If you do not, one of the numbers was typed or rounded wrongly.
Thickness examples
A sheet of paper is about 0.1 mm thick, which is about 3.94 mils. A wire of 0.5 mm is about 19.7 mils. Mils are used for coatings, plastic film and printed circuit boards.
Quick conversion answers
| Conversion | Result |
|---|---|
| 1 mm to mils | 39.37 mil |
| 0.1 mm to mils | 3.94 mil |
| 0.25 mm to mils | 9.84 mil |
| 0.5 mm to mils | 19.69 mil |
| 2 mm to mils | 78.74 mil |
| 5 mm to mils | 196.85 mil |
| 10 mm to mils | 393.7 mil |
| 25.4 mm to mils | 1,000 mil |
Where this conversion is used
Engineers, makers and packaging specifiers convert millimetres to mils for film, sheet, foil and wire thickness.
Mechanics and hobbyists convert gap and gasket sizes between metric and thou.
Buyers compare product thickness listed in millimetres against supplier sheets in mils.
Common mistakes to avoid
- Multiplying by 0.0254. That converts mils to millimetres.
- Treating mil as millimetre. A mil is about 39 times smaller.
- Using 40 for exact work. It overstates by about 1.6 percent.
- Confusing mil with micron. One mil is 25.4 µm.
Related converters
Frequently asked questions
How do I convert millimeters to mils?
Divide the millimetres by 0.0254, or multiply them by 39.3701.
What is a mil?
A mil, or thou, is one thousandth of an inch, which is 0.0254 mm.
How many mils are in a millimeter?
About 39.37 mil.
How thick is a payment card in mils?
About 30 mil, or roughly 0.76 mm.
What is the difference between mil and mm?
A mil is a thousandth of an inch and a millimetre is about 39 times larger.
How many mils are in an inch?
There are 1,000 mil in an inch.
Sources
- Wikipedia: Thousandth of an inch (en.wikipedia.org)
- NIST: SI units, length (nist.gov)
- Wikipedia: International yard and pound (en.wikipedia.org)
