Mil to mm Conversion Calculator
Enter a value in mils (thousandths of an inch) to get the exact equivalent in millimeters, or enter millimeters to convert back to mils. The conversion factor is exact: 1 mil = 0.0254 mm. Results update as you type, and the steps panel shows the full working.
Formula
Worked example
10 mil to mm: 10 × 0.0254 = 0.254 mm. Reverse: 1 mm to mil: 1 ÷ 0.0254 = 39.3701 mil.
What is a mil?
A mil is a unit of length equal to one-thousandth of an inch (0.001 in). The name comes from the Latin "mille," meaning thousand. Because 1 inch is defined as exactly 25.4 mm, one mil is exactly 0.0254 mm or 25.4 micrometers. Mils are widely used in the United States for measuring the thickness of plastic films, paint coatings, metal foils, paper, and adhesives. You will also see them in printed circuit board (PCB) design for trace widths and copper weights, and in plastic sheeting sold at hardware stores, where "10 mil" and "6 mil" labels indicate the product thickness.
How to convert mils to millimeters
The conversion is straightforward because the factor is exact: multiply the number of mils by 0.0254 to get millimeters. For example, 5 mil = 5 × 0.0254 = 0.127 mm, and 100 mil = 100 × 0.0254 = 2.54 mm. To reverse the conversion and go from millimeters to mils, divide by 0.0254 (or equivalently multiply by 39.3701). The factor 0.0254 is not an approximation: it is the exact legal definition of the inch agreed internationally in 1959, so no rounding error accumulates in either direction.
Mils vs. millimeters in practice
Millimeters are used in virtually every country except the United States for engineering drawings, machined parts, and material specifications. Mils remain common in North American plastic film and coating industries, where packaging engineers specify film gauge in mils (1 mil = about 25 microns). Converting between them is frequently needed when sourcing materials internationally, reading foreign datasheets, or importing CAD drawings. A 6 mil vapor barrier specified by a US contractor is 0.1524 mm; a European datasheet listing 0.2 mm film corresponds to about 7.87 mil.
Mils in PCB and electronics contexts
In electronics, "mil" or "thou" is the standard unit for PCB trace widths and pad spacings. A trace of 10 mil (0.254 mm) is a common minimum for standard PCB production, while fine-pitch components can require 4 mil (0.1016 mm) or even 3 mil traces. Gerber files and PCB design tools such as KiCad, Eagle, and Altium all support mil units alongside millimeters. When ordering boards from Asian manufacturers who quote dimensions in mm, converting mil specifications to mm avoids fabrication errors.
Common mil thicknesses and their mm equivalents
| Mils (mil) | Millimeters (mm) | Micrometers (um) | Typical application |
|---|---|---|---|
| 0.5 | 0.0127 | 12.7 | Thin optical coatings, PVD films |
| 1 | 0.0254 | 25.4 | Thin polyethylene wrap, packaging film |
| 1.5 | 0.0381 | 38.1 | Standard food-grade cling film |
| 2 | 0.0508 | 50.8 | Lightweight trash bags |
| 3 | 0.0762 | 76.2 | Standard grocery/produce bags |
| 4 | 0.1016 | 101.6 | Medium-duty freezer bags |
| 6 | 0.1524 | 152.4 | Heavy-duty garbage bags |
| 8 | 0.2032 | 203.2 | Industrial liner bags |
| 10 | 0.254 | 254 | Agricultural poly film, greenhouse cover |
| 12 | 0.3048 | 304.8 | Standard poly sheeting (hardware stores) |
| 15 | 0.381 | 381 | Heavy-duty construction sheeting |
| 20 | 0.508 | 508 | Contractor bags, pond liners (light) |
| 30 | 0.762 | 762 | Heavy pond/pool liners, vapor barriers |
| 40 | 1.016 | 1016 | Geomembrane liners (light) |
| 60 | 1.524 | 1524 | Standard geomembrane HDPE liner |
| 100 | 2.54 | 2540 | Heavy industrial membrane |
Reference values for plastics, coatings, films, and construction materials. 1 mil = 0.0254 mm (exact).
Frequently asked questions
How many millimeters is 1 mil?
1 mil = 0.0254 mm exactly. This follows from the definition that 1 inch = 25.4 mm and 1 mil = 0.001 inch, so 1 mil = 0.001 × 25.4 = 0.0254 mm.
How many mils are in 1 millimeter?
1 mm = 1 / 0.0254 = 39.37007874... mil. In everyday use, 1 mm is approximately 39.37 mils.
Is a mil the same as a micrometer or micron?
No. A mil (or thou) is 0.0254 mm = 25.4 micrometers. A micrometer (micron, um) is 0.001 mm. So 1 mil is 25.4 times larger than 1 micron. Confusion arises because "micron" sounds similar to "mil," but they differ by a factor of 25.4.
What is a common mil thickness for plastic sheeting?
Standard plastic sheeting sold at hardware stores is usually 3-6 mil (0.076-0.152 mm) for light-duty use, 10-12 mil (0.254-0.305 mm) for general construction, and up to 20-60 mil (0.508-1.524 mm) for heavy geomembranes and pond liners.
Why is the conversion factor 0.0254 instead of a round number?
The inch was defined in 1959 as exactly 25.4 mm by international agreement. One mil is one-thousandth of an inch, so 1 mil = 25.4 / 1000 = 0.0254 mm. The factor looks unrounded from the metric side but is perfectly clean from the imperial side (1/1000 of an inch).
What is the difference between a mil and a mil in firearms (angular mil)?
There are two entirely different units called "mil." The length unit (this calculator) is one-thousandth of an inch. The angular mil used in ballistics and artillery is an angle equal to 1/6400 of a full circle (NATO definition). They are unrelated; always check context to know which "mil" is meant.
How do I convert 20 mils to mm?
20 mil × 0.0254 = 0.508 mm. You can also think of it as 20/1000 inch = 0.020 in, then 0.020 × 25.4 = 0.508 mm.