Article — Mil Conversion Calculator
Mil Conversion — Thou to Micrometers, Millimeters, Inches
The linear mil is exactly one one-thousandth of an inch — 0.001 in, or 25.4 micrometers. It's the US industry unit for paint thickness, plastic film gauge, PCB copper, and thin material specs. To convert mils to metric, multiply by 25.4 (gives µm), 0.0254 (mm), or 0.00254 (cm). The factor is exact because the inch is defined as 25.4 mm by international agreement.
The word "mil" carries two completely different meanings depending on context. In length measurement it's a thousandth of an inch. In ballistics and optics it's an angular unit equal to 1/6400 of a circle. This article covers only the linear mil.
What is a mil?
A mil — short for "thousandth" — is 0.001 inch. The unit grew out of US machine-shop practice in the late 19th century when sheet-metal gauges, paint coatings, and electrical-insulation thicknesses all needed sub-inch precision. The word "thou" is the UK preferred form; "mil" dominates in the US.
The unit fits a useful niche. A mil is small enough to measure thin coatings (1–10 mil) and large enough that you don't need scientific notation. Most coating gauges, plastic-film measuring tools, and machinist micrometers read directly in mils with one-decimal resolution.
Before 1959, the US inch was defined as 1/39.37 m exactly (used since 1893), while the UK inch was 1/39.3700787 m. Both yielded a mil close to 25.4 µm but with slightly different last digits. The 1959 International Yard and Pound Agreement standardized the inch at exactly 25.4 mm, making the mil exactly 25.4 µm worldwide.
Mil to metric formulas
The base conversion is mil × 25.4 = µm. Every other length conversion follows from that. To get millimeters multiply by 0.0254 (= 25.4 / 1000). For centimeters multiply by 0.00254. For meters multiply by 0.0000254 (= 2.54 × 10⁻⁵).
Going the other way uses the same factor inverted. µm / 25.4 = mil. mm × 39.37 = mil. cm × 393.7 = mil. The 39.37 factor for mm-to-mil is simply 1000 / 25.4 — three decimal places of the inch-mm ratio.
Linear vs angular mil
The angular mil is 1/6400 of a full circle (NATO standard). One mil at 100 yards equals 1 inch of bullet drop or scope adjustment — a coincidence of geometry that makes ballistic computation easy. Russian and Swedish forces use slightly different definitions (1/6000 and 1/6300 respectively).
A rifle scope with mil-dot or mrad turrets adjusts in angular mils. A 1 mil click moves point-of-impact 1 inch at 100 yards, 2 inches at 200 yards, 10 inches at 1000 yards. This has zero relationship to the 0.001 inch linear mil despite identical spelling.
A "5 mil paint coat" is 0.005 in (127 µm) thick. A "5 mil scope adjustment" is roughly 5 inches of point-of-impact shift at 100 yards. Same word, completely different physical meaning. Reading the source document carefully matters.
Mil in coating specifications
Architectural paint manufacturers spec dry film thickness (DFT) in mils. A single coat of latex applied at 350 ft²/gal gives roughly 4.6 mil wet, drying to about 1.5 mil DFT at 33 percent solids. Two coats reach 3 mil DFT — the minimum for full-spectrum opacity over a primer.
Industrial coatings push thicker. Marine antifouling paints run 4 mil per coat with 2–3 coats. Tank linings hit 30–60 mil. Industrial floor epoxies are 8–15 mil. These all measure with ASTM D7091 magnetic or eddy-current gauges referenced to NIST-traceable standards.
Mil coverage shortcut: 1 mil DFT at 100 percent solids covers 1604 ft²/gal of paint. So a 2-mil coating at 60 percent solids needs 2 / 0.6 = 3.33 mil wet, covering 1604 / 3.33 = 482 ft²/gal of wet paint.
Mil in plastic film and PCBs
Plastic film and bag specs run from 0.5 mil (light supermarket bag) to 20+ mil (pool liners, heavy tarps). The plastic industry also uses "gauge" = mil × 100, so a 4-mil sheet is a 400-gauge sheet. Both numbers appear on product packaging.
PCB copper is sold by mass per area but understood as mil thickness. 1-oz copper is 1.4 mil (35 µm), the default for signal traces. 2-oz copper (2.8 mil, 70 µm) handles higher current. Heavy power conversion uses 3-oz, 4-oz, even 6-oz copper. The fabricator's design rules give minimum trace widths in mils.
- 1 mil = 25.4 µm (paint, fine plastic bag)
- 1.4 mil = 35 µm (PCB 1 oz copper)
- 2 mil = 50.8 µm (food gloves, packing tape)
- 3 mil = 76 µm (heavy trash bag, anodize coat)
- 4 mil = 102 µm (copy paper, contractor sheet)
- 6 mil = 152 µm (vapor barrier, lamination)
- 10 mil = 254 µm (pool cover, heavy tarp)
- 20 mil = 508 µm (marine paint)
- 50 mil = 1.27 mm (heavy industrial coating)
Mil conversion table
The mil-to-metric factor is exact, so any table value is the rounded display of an exact rational number. The table at the right shows mil values and equivalents for the most-quoted thicknesses in coatings and packaging.
Measuring mil thickness
Magnetic-pull gauges measure ferrous-substrate coatings (paint on steel) up to 6 mil DFT. Eddy-current gauges read on aluminum and other non-ferrous metal. Ultrasonic gauges work on plastic, wood, fiberglass — any non-conductive substrate. Modern combination gauges switch automatically.
For free films (plastic sheets not attached to a substrate), a digital micrometer with anvil and spindle reads directly to ±0.1 mil resolution. Calibration against NIST-traceable gauge blocks is required for QC documentation. ASTM D6988 covers plastic film thickness measurement.
Common mil-conversion mistakes
The headline mistake is reading "mil" as "mm." A 5-mil coating is 0.127 mm, not 5 mm — a 40x difference. Catalog typos and translation tools sometimes drop the "i" and produce "5 mm" when the source said "5 mil." Sanity-check by visual scale: 5 mil is about the thickness of a sheet of copy paper (~4 mil); 5 mm is a stack of 40 business cards.