Mils to Millimeters Converter

What Is the Mils to Millimeters Converter?

A mil -- one thousandth of an inch, also called a "thou" -- is the standard unit for specifying thin-material thickness in US manufacturing: plastic film, wire coatings, laminates, and PCB copper layers are all commonly quoted in mils. This converter turns a mil measurement into millimeters, the unit most metric-based specs and measuring tools actually use.

How to Use This Converter

Enter your measurement and select whether it's currently in Mils or Millimeters from the "From Unit" dropdown. Both results update instantly.

The Conversion Formula

One mil is defined as exactly one thousandth of an inch, and one inch is exactly 25.4mm -- so one mil is exactly 0.0254mm. This is a precise, exact conversion, not a rounded approximation.

Worked Examples

Common Values Reference

Practical Uses

PCB designers converting a mil-based trace-width or copper-thickness spec into millimeters for a metric CAD tool. Packaging buyers checking a US mil-based film spec against a metric thickness requirement.

Important Notes

Worked Examples

The conversion is a simple multiplication by 0.0254, but seeing it with several typical values makes it easy to develop a feel for the sizes involved.

• 3 mils: 3 times 0.0254 is 0.0762 mm.
• 4 mils: 4 times 0.0254 is 0.1016 mm.
• 15 mils: 15 times 0.0254 is 0.381 mm.
• 62 mils: 62 times 0.0254 is 1.5748 mm, which is close to the common 1.6 mm circuit board thickness people often meet.
• 500 mils: 500 times 0.0254 is 12.7 mm, exactly half an inch.

Step-by-Step Usage

First, confirm the number in your specification is really in mils and not in a different unit such as microns or points. Second, enter it and check that the From Unit reads Mils. Third, read the millimeter result and decide how many decimal places you need. For most everyday comparisons two or three decimals is plenty, while for tight manufacturing tolerances you should keep more. Finally, if you will send the value to a supplier, write both units side by side, such as 10 mils (0.254 mm), so that nobody has to guess.

Common Mistakes

• Confusing a mil with a millimeter. A millimeter is about 39 times larger, so mixing them up creates a huge error.
• Mistaking mils for microns. A mil is 25.4 microns, so a 1 mil film is 25.4 microns thick. A micron is a much smaller unit than a mil.
• Rounding too soon. Rounding 0.0762 mm to 0.08 mm in an early step makes the final answer about 5 percent higher than it should be.
• Reading the mil as a mile abbreviation, or as the Scandinavian unit of 10 kilometers, which is unrelated.

Where the Numbers Show Up

In practice you meet mils in packaging film, adhesive tape, pool liners, heavy plastic sheeting, protective screen films, wire insulation, and PCB layouts. Because tolerances are printed in the same unit as the nominal value, remember to convert the tolerance as well as the main number. A spec of 10 mils with a tolerance of 1 mil becomes 0.254 mm plus or minus 0.0254 mm. For safety-critical or regulated applications, always work from the manufacturer's official documentation rather than a converted approximation.

Frequently Asked Questions