mm to mil Converter (PCB)
Convert millimetres to mils and back, with the trace widths and clearances the numbers actually map to.
1 mil = 0.0254 mm. 1 mm = 39.3701 mil. Mils are thousandths of an inch (also called thou) — not millimetres. This is the mix-up that ruins boards.
Named for PCB use: fab minima, trace widths, clearances, drill sizes, and stackup thicknesses are typically quoted in mils in the US and in mm elsewhere. EDA tools accept either — mixing them silently passes DRC.
Common PCB values
| Millimetres | Mils | Where it shows up |
|---|---|---|
| 0.1 mm | 3.94 mil | Fine-pitch escape trace |
| 0.127 mm | 5 mil | Common minimum clearance |
| 0.15 mm | 5.91 mil | Advanced-spec trace/space |
| 0.2 mm | 7.87 mil | Typical low-cost fab minimum |
| 0.25 mm | 9.84 mil | Default signal trace |
| 0.5 mm | 19.69 mil | Light power trace |
| 1.0 mm | 39.37 mil | Power rail |
| 1.6 mm | 62.99 mil | Standard board thickness |
| 2.54 mm | 100 mil | Classic 0.1 in header pitch |
Why both units exist in PCB design
PCB fabrication grew up in the United States, so trace widths, clearances and drill sizes are quoted in mils, while component packages and board outlines are metric. Most EDA tools let you mix them, which is exactly how a 0.2 mm clearance gets entered where 0.2 mil was meant — a 127× error that passes DRC silently if the rule was set in the wrong unit too.
Frequently asked
- How many mils in a millimetre?
- 1 millimetre equals 39.3701 mils. A mil is one thousandth of an inch (0.001 in = 0.0254 mm).
- Is a mil the same as a millimetre?
- No — this is the most common PCB error. 1 mil (0.0254 mm) is roughly 1/39 of a millimetre. Mixing them causes silent 25×–127× design errors.
- What is 0.1 mm in mils?
- 0.1 mm = 3.937 mils. Often written as 4 mil in fab-capability sheets.
- What is 6 mil in mm?
- 6 mil = 0.1524 mm. A typical low-cost fab minimum trace/space.
Converting is the easy part; the board still has to respect those numbers. PCBEditor checks every trace and gap against your fab's limits while it routes.
Design a PCB with these units →Related free tools
- Mil to mm Converter for PCB SpecsConvert mils to millimetres one value at a time or a whole fab capability sheet at once, ready to type into metric design rules.
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