Mil to MM Conversion

Article — Mil to MM Conversion

Mil to mm: a precise conversion for thousandths of an inch

One mil equals exactly 0.0254 millimeters. A mil — also called a thou in the UK — is one-thousandth of an inch (0.001 in). The factor is exact since 1959, when the international yard-and-pound agreement fixed the inch at exactly 25.4 mm.

The mil is the working unit of PCB designers, coating inspectors and plastic-film engineers. It survives in fields where measurements live below a millimeter but above a micron — the range where decimals stop being convenient and ratios of small integers start to matter. PCB traces of 6 and 8 mils are easier to think about than 0.1524 and 0.2032 mm.

What is a mil

A mil is 1/1000 of an inch, full stop. The name traces back to the Latin mille, meaning thousand — the same root that gave us "mile" and "millimeter". British engineers use "thou" (short for thousandth) to mean the same thing and avoid confusion with the metric prefix. Both are in active use.

The unit dates to the early 19th century, when American machine-tool builders needed sub-inch precision in a system where the smallest legal unit was the inch. The 1/1000 division was a natural choice, since decimal subdivision was already standard for surveying and engineering drawings.

The mil-to-mm formula

Multiply mils by 0.0254 to get millimeters. To go the other way, divide by 0.0254 (or multiply by 39.3700787). The factor comes from the inch-to-millimeter conversion (1 inch = 25.4 mm exactly), divided by 1000 to scale down to mils.

The number is exact. The 1959 International Yard and Pound Agreement defined the inch as 25.4 mm precisely — not approximately. This means a CAD package that stores PCB pad sizes in mils and exports in millimeters can do so without any precision loss whatsoever. The conversion is rational; both directions terminate.

Mil vs. millimeter vs. milliradian

Three "mil-" units cause regular confusion. A mil of length is 0.0254 mm. A millimeter is 1 mm. A milliradian is an angle, equal to 1/1000 of a radian — used in military targeting, optics, surveying and firearms. These are not interchangeable, and context never makes the meaning obvious.

The clue is the field. PCB and coating documents use "mil" for length; firearm scope reticles use "mil" for angle. A 1-mil rifle adjustment moves the point of impact about 3.6 inches at 100 yards — a milliradian conversion, nothing to do with thousandths of an inch. When a datasheet says "trace width 8 mil," it is length. When a spec says "ranging accuracy to 0.1 mil," it is angle.

Mil to mm in PCB design

Printed-circuit-board houses still quote trace widths, pad sizes and clearances in mils. Industry standards from the IPC (Association Connecting Electronics Industries) — IPC-2221 for general design, IPC-A-600 for acceptability — publish tables in mils. A 5-mil minimum trace width is a common rule for cheap-and-cheerful prototyping; high-density designs go to 3 mil or below.

Copper weight is the other place mils show up. PCBs are specified by copper "ounces per square foot", a holdover from electroplating practice. One ounce per square foot deposits a 1.37-mil (35-µm) copper layer. Two-ounce boards double that to 2.74 mil and are used where current-carrying or thermal mass matters — power supplies, motor drivers, and high-current automotive electronics.

Mils in paint, plating and films

Coatings, paints and plastic films use mils almost universally in the US market. A factory automotive paint job is 4–6 mils thick across all layers (primer + base coat + clear coat). Powder coats applied to railings and outdoor furniture run 2–10 mils. Marine epoxies for boat hulls can exceed 20 mils per coat.

Plastic films span a wider range. Kitchen plastic wrap is 0.5 mil thick (12.7 microns). A heavy-duty trash bag is 3 mil. Pond liners are typically 45 mil EPDM rubber. Greenhouse polyethylene comes in 4-mil and 6-mil grades, with the 6-mil rated for a 4-year UV life. Field gauges measure film thickness in mils, with dial graduations in 0.1-mil steps.

Mil to mm quick conversions

For mental math, treat the mil-to-mm factor as 0.025. A 100-mil trace is about 2.5 mm; the exact answer is 2.54. Going the other way, multiply mm by 40: 1 mm ≈ 40 mils (exact: 39.37). The 1.6% error in both shortcuts is small enough for verbal discussion but not for design files.

Half-precision shortcuts: 4 mil ≈ 0.1 mm. 40 mil ≈ 1 mm. 400 mil ≈ 10 mm (one centimeter). These round-numbered relationships are why some engineers swap between systems freely in conversation but always work in one system inside CAD.

• 1 mil = 0.0254 mm = 25.4 µm
• 10 mil = 0.254 mm (a thick paint layer)
• 62 mil = 1.57 mm (standard FR-4 PCB)
• 100 mil = 2.54 mm (one DIP pin spacing)
• 1 mm = 39.37 mil (more than 1.5 DIP pitches)
• 1 oz copper = 1.37 mil = 34.8 µm
• 2 oz copper = 2.74 mil = 69.6 µm
• Kitchen plastic wrap = 0.5 mil = 12.7 µm

Common mil-to-mm mistakes

The single most common error is confusing mil with millimeter. A 6-mil trace is 0.15 mm, not 6 mm — a factor-of-40 mistake. PCB fabrication software will reject the design, but coating specifications can pass through unflagged. Always check whether the source uses "mil" or "mm".

The second is over-rounding the factor. Spreadsheets sometimes store 0.025 instead of 0.0254, a 1.6% error. On a 100-mil dimension that is 4 microns — invisible to the eye, but enough to fail tight-tolerance inspection. Use 0.0254 (or the full 0.025400) as a stored constant, never as a literal in repeated cells.

The third is mil-vs-milliradian in mixed-discipline documents. Defense and aerospace specifications routinely mix the two within a few pages. A "tolerance of 5 mil" on a mechanical drawing means 0.127 mm; the same wording on an optical-alignment spec means 5 milliradians of angular deviation. Always confirm the field convention before scaling.