PCB Design · Units
mm to mils Conversion Guide for PCB Designers
PCB designers constantly flip between metric and imperial units. Component datasheets and modern footprints are usually metric (mm), but fab houses, legacy libraries, and trace/clearance rules are still quoted in mils. Here is a quick reference, the math, and an interactive calculator.
Exact: 1 mil = 0.0254 mm · 1 mm ≈ 39.3701 mils
What is a mil?
A mil is one-thousandth of an inch (0.001 in). It is not a millimeter. In PCB design, mils are the traditional unit for trace widths, clearances, annular rings, and drill sizes — for example "6/6" means a 6 mil trace with 6 mil clearance.
The conversion
1 mil = 0.0254 mm(exact)1 mm = 1 / 0.0254 ≈ 39.3701 milsmm → mils: multiply by 39.3701mils → mm: multiply by 0.0254
Why designers switch units
Most modern parts (0402, QFN, BGA) have metric pitches like 0.4 mm or 0.5 mm. But fab capability sheets, older through-hole parts (0.1″ headers), and a lot of routing guidance still use mils. EDA tools let you display either unit, so reading and writing both fluently saves a lot of mental arithmetic — and prevents off-by-an-order-of-magnitude mistakes when you accidentally paste a mm value into a mils field.
Common PCB values
| mm | mils | Typical use |
|---|---|---|
| 0.1 | 3.937 | Fine trace / fine pitch |
| 0.127 | 5 | 5 mil rule on most fabs |
| 0.15 | 5.906 | — |
| 0.2 | 7.874 | Typical signal trace |
| 0.2032 | 8 | — |
| 0.254 | 10 | 10 mil — common default |
| 0.3 | 11.811 | — |
| 0.381 | 15 | — |
| 0.5 | 19.685 | — |
| 0.508 | 20 | 20 mil — common power trace |
| 0.635 | 25 | — |
| 0.762 | 30 | — |
| 1 | 39.37 | — |
| 1.27 | 50 | 0.05" pitch headers |
| 2.54 | 100 | 0.1" pitch — DIP / breadboard |
Rules of thumb
- 0.1 mm ≈ 4 mils — useful for quick estimates of clearance and trace width.
- 0.25 mm ≈ 10 mils — a comfortable default trace for signals on hobbyist 2-layer boards.
- 1 mm ≈ 40 mils — power traces and big copper.
- 2.54 mm = 100 mils exactly — the classic 0.1″ pitch for DIP and pin headers.
- 1.27 mm = 50 mils exactly — SOIC pin pitch and 0.05″ headers.
FAQ
- Is a mil the same as a millimeter?
- No. 1 mil = 0.0254 mm. A 10 mil trace is only 0.254 mm wide.
- What is the smallest trace most fabs make?
- The widely available baseline is ~5 mil (≈0.127 mm) trace and clearance. Cheaper tiers cap at 6 mil; advanced HDI processes can go below 3 mil.
- Why are drill sizes often in mils?
- Drill bits were historically tooled in imperial sizes, and Gerber/Excellon files have decades of legacy in mils. Modern CAM tools accept both.
Designing a board now? Try the free in-browser editor — switch the grid between mm and mils with one click.