Mil Conversion Calculator
Enter any length or thickness in mils, inches, millimeters, micrometers, centimeters, or meters, and this calculator instantly converts it to all other units. A mil is exactly one-thousandth of an inch (0.001 in) and is the standard unit for measuring thin materials like PCB laminates, plastic films, paint coatings, and adhesive tapes.
What is a mil?
A mil is a unit of length equal to one-thousandth of an inch: 1 mil = 0.001 in = 0.0254 mm = 25.4 micrometers. The name comes from the Latin "mille" (thousand). It is also called a "thou" in British engineering tradition. Despite sharing a similar name, a mil is completely different from a millimeter: 1 millimeter is approximately 39.4 mils, so the two units are about 40 times apart. Confusing mils and millimeters is one of the most common measurement errors in coating inspection and PCB design.
Where mils are used
Mils are the standard thickness unit across several industries in North America. In the coatings industry, paint inspectors measure dry film thickness (DFT) in mils using instruments like a Positector or Elcometer gauge. Specifications typically call for 2-6 mils of dry film, which corresponds to a much thicker wet film applied by brush or roller. In printed circuit board manufacturing, copper foil weight is expressed in ounces per square foot, but the resulting thickness is quoted in mils: 1 oz copper = 1.37 mils, 2 oz = 2.74 mils. Plastic films - trash bags, food wrap, greenhouse poly, geomembranes, and vapor barriers - are all specified and sold in mils. Adhesive tapes, labels, and packaging laminates use mils for their face stock and adhesive layer thicknesses.
Mil vs millimeter vs micrometer
Precision measurement involves three similar-sounding but very different units. A millimeter (mm) is 1/1000 of a meter. A micrometer (um, also called a micron) is 1/1000 of a millimeter, or 1/1,000,000 of a meter. A mil is 1/1000 of an inch, which equals 25.4 micrometers, or 0.0254 millimeters. To put it another way: 1 mm = 39.37 mils, and 1 mil = 25.4 microns. In everyday coatings work, dry film thickness gauges often read in both mils and micrometers, switching between them with the understanding that the numeric value in microns is roughly 25 times the value in mils for any given measurement.
How to convert mils to other units
All mil conversions flow from one exact definition: 1 inch = 25.4 mm (exact, by international agreement since 1959), and 1 mil = 0.001 inch. From this: mils to millimeters - multiply by 0.0254; mils to inches - divide by 1000; mils to micrometers - multiply by 25.4; mils to centimeters - multiply by 0.00254; mils to meters - multiply by 0.0000254. To go the other way: inches to mils - multiply by 1000; millimeters to mils - multiply by 39.3701; micrometers to mils - divide by 25.4.
Common material thicknesses in mils
| Material / Application | Thickness (mils) | Thickness (mm) | Notes |
|---|---|---|---|
| Human hair (average) | 2.5 - 3.5 | 0.064 - 0.089 | Reference only |
| Gold plating on PCB contacts | 0.1 - 0.3 | 0.0025 - 0.008 | Hard gold flash |
| Conformal coating (thin) | 1 - 3 | 0.025 - 0.076 | IPC-CC-830B |
| Dry paint film (typical) | 2 - 6 | 0.051 - 0.152 | Interior/exterior wall paint |
| PCB solder mask | 0.4 - 1.2 | 0.010 - 0.030 | Epoxy liquid photoimageable |
| PCB copper foil (1 oz) | 1.4 | 0.0356 | 34 g/m2 standard |
| PCB copper foil (2 oz) | 2.8 | 0.0711 | 68 g/m2 |
| Cling wrap / food film | 0.4 - 0.6 | 0.010 - 0.015 | LDPE film |
| Zipper storage bag (thin) | 2 | 0.051 | Standard sandwich bag |
| Contractor garbage bag | 3 - 6 | 0.076 - 0.152 | Heavy-duty HDPE |
| Vapor barrier (standard) | 6 | 0.152 | Code minimum in many regions |
| Vapor barrier (heavy) | 10 - 20 | 0.254 - 0.508 | Crawl space / radon barrier |
| Duct tape thickness | 7 - 12 | 0.178 - 0.305 | Cloth-backed pressure tape |
| Credit card | 30 | 0.762 | ISO/IEC 7810 ID-1 |
Typical thickness values encountered in manufacturing, electronics, and coatings. 1 mil = 0.001 in = 0.0254 mm.
Frequently asked questions
Is a mil the same as a millimeter?
No. A mil is one-thousandth of an inch (0.001 in = 0.0254 mm), while a millimeter is one-thousandth of a meter (1 mm = 39.37 mils). They sound similar but are about 39 times different in magnitude. Mixing them up is a common and costly error in coatings specifications and PCB fabrication.
What is the difference between a mil and a thou?
They are exactly the same unit. "Mil" is the American English term; "thou" is the British engineering shorthand for thousandth of an inch. Both equal 0.001 in = 25.4 micrometers. In everyday use, machinists in the UK and Australia often say "thou" while North American industry uses "mil."
How many mils is a millimeter?
1 millimeter equals exactly 39.3700787 mils (to seven decimal places). In practical terms, most engineers round this to 39.37 mils per millimeter.
How do I convert mils to microns (micrometers)?
Multiply the number of mils by 25.4 to get micrometers. For example, 3 mils = 3 x 25.4 = 76.2 micrometers. To go the other way, divide microns by 25.4 to get mils.
Why do paint coatings use mils instead of millimeters?
Paint coatings in North America have used mils since the early twentieth century because the values are conveniently sized: typical protective coatings are 2-10 mils thick, while the same measurements in millimeters would be awkward fractions like 0.051 to 0.254 mm. The coating inspection industry, instruments, and specifications all standardized on mils, so the unit persists for consistency across manufacturers, contractors, and standards bodies such as SSPC, NACE, and ISO.
What thickness is 6 mil plastic sheeting?
6 mil plastic = 0.006 inches = 0.1524 mm = 152.4 micrometers. It is a common vapor barrier used under concrete slabs and in crawl spaces. By comparison, standard cling wrap is about 0.5 mil, while contractor garbage bags are 3-6 mil.