Length Unit Converter (mm, mil, inch, μm)

PCB work runs on two incompatible measurement systems at once, and the boundary between them is where expensive mistakes happen. North American fabricators quote in mils; the rest of the world quotes in millimetres; datasheets mix both freely; and the two units differ by a factor of roughly 39.4 while sounding almost identical.

The conversion rests on one exact definition. Since the 1959 international yard and pound agreement:

Everything else follows without rounding. A mil is one thousandth of an inch, so 1 mil = 0.0254 mm = 25.4 μm, and 1 mm = 1000/25.4 = 39.3701 mil. British usage calls the same unit a thou; it is identical to a mil.

Why copper is measured in ounces

Copper foil thickness is quoted as a weight per unit area: one ounce of copper spread evenly over one square foot. Working that through gives roughly 34.8 μm (1.37 mil) for 1 oz, which is the figure this site uses throughout. Pure mass density would put it at 34.06 μm and IPC-4562A nominal is 34.29 μm, but foil tolerance is around ±10%, so the choice between those conventions is far from the dominant uncertainty. The convention survives because foil is bought by weight, not by thickness.

The practical figures worth memorising:

Numbers worth knowing by heart

• 4 mil ≈ 0.1 mm — a typical fine-line trace or space
• 8 mil ≈ 0.2 mm — a common default trace width
• 10 mil = 0.254 mm — a standard thin core
• 62 mil ≈ 1.6 mm — the standard board thickness
• 100 mil = 2.54 mm — classic through-hole pin pitch

That last one is the origin of the imperial grid the whole industry inherited: 0.1 inch pin spacing, which is why so many legacy footprints land on awkward metric numbers.