Convert mil to mm

A mil is short for milli-inch: one thousandth of an inch.

mil

0.1016mm

History

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Formula

Formula: mm = mil × 0.0254

Design notes

1 mil = 1/1000 inch = 0.0254 mm. The name is a contraction of milli-inch and it is also called a thou. It has nothing to do with the metric prefix milli, so take care not to confuse it with a millimetre.

Schematic and PCB CAD tools default to different units, so this conversion comes up constantly when handing design data over or writing fabrication drawings.

A connector on a "100 mil pitch" is the same as a 2.54 mm pitch.

When you need this

For turning a fabricator's specification or an overseas datasheet quoted in mils into the millimetres your CAD tool uses. Requirements like "4mil minimum trace" or "5mil minimum annular ring" have to become design rules.

Where mils appear in fabrication specs

Minimum trace width and spacing. Quoted as 4mil, 5mil or 6mil. These form the backbone of your design rules.
Annular ring. The width of pad remaining outside the drilled hole on one side. Below 5mil (0.127mm) drill registration error risks breaking out of the pad.
Solder mask clearance. Usually specified as 2–3mil.
Board thickness. 62mil = 1.6mm, 31mil = 0.8mm, 93mil = 2.4mm are the standard options.
Copper weight is quoted in oz, not mils — see Copper weight (oz) to thickness.

Conversion table

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.

Frequently asked questions

What is annular ring?

The pad outer diameter minus the hole diameter, divided by two — the width of copper remaining on one side. It is the margin against drill registration error. 5mil (0.127mm) or more is standard; below that, a misregistered drill can break out of the pad and lose connection.

How many mils is 1oz copper?

About 1.4mil (0.035mm). Copper is conventionally specified in oz rather than mils, deriving from the weight per square foot. Use Copper weight (oz) to thickness to get millimetres.

What if a specification uses decimal inches?

A figure like 0.005" is 5mil — multiply by 1000. Converting to mils first and then to millimetres tends to cause less confusion.

Standards and references

IPC-2221B — Generic PCB design standard.
IPC-6012 — Board performance specification; annular ring requirements.

Last updated: 2026-08-29