Mils, Millimetres, and the PCB Unit Mess

Why PCB design has a unit problem

The North American PCB industry grew up in imperial units. Drill sizes are in mils (thousandths of an inch). Pad sizes are in mils. The IPC standards that define everything from annular ring to conductor spacing were written in mils.

The rest of the world uses metric. Component packages are defined in millimetres. Board outlines are in millimetres. Pick-and-place machines work in millimetres.

So every PCB designer works in both systems, converting constantly, and getting it wrong occasionally. A 10 mil trace and a 10 mm trace differ by a factor of 254. Nobody confuses them visually on a board, but in a calculation or a script, the wrong factor is silent.

The conversions

Exactly:

• 1 inch = 25.4 mm (by definition since 1959)
• 1 mil = 0.0254 mm = 25.4 µm
• 1 mm = 39.3701 mil

For quick mental conversion:

• Mils to mm: divide by 40, subtract 1.5%. Good to 0.1% for anything under 500 mil.
• mm to mils: multiply by 40, add 1.5%.

Or remember: 100 mil = 2.54 mm. Everything scales from there.

The one constant worth memorising

1 oz/ft² of copper = 34.8 µm = 1.37 mil thick.

More precisely: 0.0348 mm. This is the number that connects copper weight (an area-density specification) to copper thickness (a geometric dimension).

Where it comes from: one ounce of copper spread over one square foot, given copper's density of 8,960 kg/m³, gives a sheet 34.79 µm thick. The derivation:

The commonly used value of 35 µm (or 1.4 mil) is a rounded approximation. For impedance calculations where copper thickness matters — stripline with heavy copper, or CPW with tight gaps — use 34.8 µm per ounce.

Common weights:

Where the unit mess actually bites

Impedance calculations

Trace width in mils, dielectric height in mm (from the laminate datasheet), copper thickness in oz. Three different units in one formula. Miss one conversion and your impedance is off by an order of magnitude, but the number might still look reasonable — 35 Ω instead of 50 Ω, say — because the formula is well-behaved.

Always convert everything to one system before calculating. Millimetres is usually cleanest because laminate datasheets are metric.

IPC-2152 current tables

The tables are in mils² for cross-sectional area and °C for temperature rise. If your design tool reports trace width in mm, you need to convert both width and thickness to mils before entering the table. The area in mils² is:

A 0.25 mm trace at 1 oz: that's 9.84 mil wide × 1.37 mil thick = 13.5 mil². In the IPC table, that carries about 0.7 A at 10 °C rise on an inner layer.

Drill sizes

Fabricators quote drill sizes in mils (US) or millimetres (Asia/Europe). A "10 mil" via is 0.254 mm. A "0.2 mm" via is 7.87 mil. These are close enough to confuse. If your drill table mixes sources, double-check every entry.

Finished hole size is smaller than drill size by the plating thickness on each side — typically 25 µm (1 mil) per side, so 50 µm (2 mil) total diameter reduction.

DRC spacing

Design rules in mil-based tools: 5 mil space = 0.127 mm. Metric tools might show 0.13 mm, which is actually 5.12 mil — technically different, never practically different, but it causes spurious DRC violations if the engine compares at sub-micron precision.

Pick one system for your design rules and stick with it. Don't mix.

Microns in semiconductor-adjacent work

Anything involving die-level routing, wire bonding, or wafer-level packaging lives in micrometres. The conversion to mils:

• 1 µm = 0.03937 mil
• 1 mil = 25.4 µm

A 25 µm bond wire is about 1 mil. A 5 µm metal layer is 0.2 mil. These numbers matter for current density calculations in package substrates and interposers.

Practical advice

1. Set your EDA tool to metric (mm). Component footprints are metric, board outlines are metric, and fab drawings are metric. The imperial legacy lives in reference material, not in your layout.
2. Keep a conversion constant in your scripts. Don't hardcode 25.4 in twelve places. Define it once.
3. State units explicitly in all documentation. "Trace width: 6" is ambiguous. "Trace width: 6 mil" is not. "Trace width: 0.15 mm" is not.
4. For copper thickness, use the exact constant. 34.8 µm/oz, not 35. The 0.6% error propagates into impedance and fusing calculations.

The length units calculator converts between all four PCB-relevant systems (mm, mil, inch, µm) and includes the copper weight to thickness conversion.