Convert mm to mil

Convert millimetres to mils. 1 mil = 1/1000 inch = 0.0254 mm.

mil

39.3701mil

History

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Formula

Formula: mil = mm ÷ 0.0254

Design notes

In printed circuit board design, trace widths and via diameters are often specified in mils (1 mil = 1/1000 inch). US EDA tools and component datasheets in particular tend to use mil throughout.

The conversion comes up whenever you write fabrication notes for the board house or check a Gerber file against a specification given in the other unit.

When you need this

For checking that traces you drew in millimetres satisfy the fabricator's minimums, which are quoted in mils. Typically used to sanity-check a layout before running the full design rule check.

Why PCBs are dimensioned in mils

Printed circuit boards developed in the United States, so the dimensional system is inch-derived. A mil is 1/1000 inch, and much of the field is still specified that way: component pitches (100mil for DIP and pin headers), board thickness (62mil = 1.6mm), and minimum feature specifications.

Design work outside the US is normally done in millimetres, so the manufacturing specification and the design data are in different unit systems as a matter of course. Check both before releasing to fabrication.

Values used in board design

Dimensions that come up constantly in PCB work. 1 mil = 1/1000 inch = 0.0254 mm.
mil	mm	What it means on a board
3 mil	0.076 mm	Minimum width/space on dense boards (premium)
4 mil	0.102 mm	Practical floor for high-density work
5 mil	0.127 mm	Standard minimum width/space
6 mil	0.152 mm	Comfortably within cheap fabrication
8 mil	0.203 mm	Relaxed design
10 mil	0.254 mm	Ordinary signal trace
20 mil	0.508 mm	Light power trace
40 mil	1.016 mm	Power trace
62 mil	1.575 mm	Standard 1.6mm board thickness

Fabrication specifications are usually written in mils while the CAD work is in millimetres. Remembering just 5 mil = 0.127mm and 10 mil = 0.254mm covers most situations mentally.

Common mistakes

Confusing mil and mm and designing something 25.4 times too big. It sounds absurd, but it happens with imported component libraries. Always eyeball footprint outlines for plausibility.
Assuming "mil" is short for millimetre. Documents from different regions use the term differently. Judge from the magnitude of the number.
Rounding through a manufacturing limit. A 0.1mm trace is 3.94mil, which just misses a 4mil minimum. At the boundary, check before rounding.

Frequently asked questions

Are mil and thou the same?

Yes. Both mean 1/1000 inch. British sources tend to say thou, American sources mil, but there is no difference.

What minimum trace width should I design to?

For ordinary volume production, 5mil (0.127mm) or above is buildable by most fabricators at no premium. 4mil is increasingly supported but yields lower and shows in the price. Below 3mil is high-density territory and costs significantly more.

What is 100mil pitch in millimetres?

2.54mm. It is the standard pitch for DIP packages, pin headers and prototyping board, and the most commonly encountered dimension in electronics. Half of it, 50mil, is 1.27mm and is the SOP package pitch.

Standards and references

IPC-2221B — Generic PCB design standard; conductor width and spacing.
IPC-2615 — Board dimensions and tolerancing conventions.

Last updated: 2026-08-29