Micron to Mil Converter

Article — Micron to Mil Converter

Micron to Mil Conversion — µm to Thou (Coating & Film Thickness)

One mil equals exactly 25.4 micrometers. The mil — short for "thousandth" — is one-thousandth of an inch. Since the inch is defined as exactly 25.4 mm by international agreement, the µm-to-mil factor inherits perfect precision. Divide microns by 25.4 to get mils, multiply mils by 25.4 to get microns.

The mil dominates US specifications for paint film thickness, plastic sheet, PCB copper, and adhesive tape. Outside the US, the same measurements use microns. Both units describe the same physical thickness — the choice is industrial culture, not physics.

What is a mil?

A mil (also called a thou, short for thousandth) is one one-thousandth of an inch — 0.001 in. The unit grew out of US engineering practice in the early 20th century when fine machining tolerances and thin coatings needed a smaller-than-inch measure that still felt imperial.

The mil is unrelated to the military "mil" used in artillery and rifle scopes. That mil is an angular measurement — 1/6400 of a circle in NATO usage, 1/6000 in Russian. Confusion is common because both notations omit any unit suffix; context tells you which one is meant. A "10-mil paint coat" is thickness; a "10-mil rifle adjustment" is angle.

Micron to mil formula

Divide micrometers by 25.4. So 100 µm = 100 / 25.4 = 3.937 mil. Going the other way, multiply mils by 25.4: 5 mil × 25.4 = 127 µm. The formula has no measurement uncertainty because both units derive from the SI meter through exact rational factors.

For mental math, the approximation 25 µm ≈ 1 mil works for field estimates. The error is 1.6 percent — fine for visual inspection but not for QC documentation. The exact factor 25.4 is short enough to memorize directly.

Mil thickness in paint coatings

Coating specifications quote dry film thickness (DFT) in mils almost universally in North America. A single coat of architectural latex paint runs 1.0–1.5 mil DFT (25–38 µm). A 2-coat exterior system reaches 2.5–4 mil (64–100 µm). Industrial primers and topcoats run 2–4 mil per coat.

Heavy-duty industrial coatings push much thicker. Tank linings for chemical service can be 30–80 mil (760–2030 µm) of epoxy. Marine ablative bottom paint is 20–30 mil. Floor coatings for warehouse traffic are 15–30 mil. All measured against an ASTM D7091 reference standard with a magnetic or eddy-current gauge.

Mil in plastic film and PCBs

Plastic film thickness is the most common mil measurement consumers encounter. A grocery bag is 0.5–1 mil polyethylene (13–25 µm). A heavy contractor trash bag is 3 mil (76 µm). A pool cover is 8 mil (200 µm). Vapor barrier sheets for construction are 6 or 10 mil.

PCB copper thickness uses an unusual unit — mass per area — that translates to mils. Standard "1 ounce copper" means 1 oz of copper plated over 1 ft² of board, which works out to 1.4 mil (35 µm). "2-ounce copper" is 2.8 mil (70 µm), used for higher current capacity. Power converters can use 6-oz copper (8.4 mil, 210 µm).

• 0.5 mil = 12.7 µm (sandwich bag)
• 1 mil = 25.4 µm (single coat latex paint)
• 2 mil = 50.8 µm (typical PCB copper, food-service gloves)
• 3 mil = 76.2 µm (heavy duty trash bag)
• 5 mil = 127 µm (2-coat epoxy paint, lamination pouch)
• 6 mil = 152 µm (construction vapor barrier)
• 10 mil = 254 µm (pool liner edge, heavy tarp)
• 20 mil = 508 µm (marine antifouling paint)
• 30 mil = 762 µm (industrial floor coating)

Micron to mil quick table

Round-number micron values translate to slightly-irrational mils because the factor 25.4 doesn't divide cleanly. Useful for both directions:

Mil vs mm vs gauge

Three thickness units sometimes appear on the same spec sheet. Mils and micrometers convert through 25.4. Mils and millimeters convert through 39.37 (1 mm = 39.37 mil). The plastic-film "gauge" is mil × 100 — so 4 mil = 400 gauge.

Sheet-metal gauge is an entirely separate scale where higher gauge means thinner metal — 16 gauge steel is 1.52 mm (60 mil), 22 gauge is 0.79 mm (31 mil). The progression is inverse and depends on the metal: a 22 gauge steel sheet, brass sheet, and copper sheet all have different actual thicknesses. Don't apply plastic-film "gauge" logic to sheet-metal gauge.

Measuring mil thickness

Three field gauge types dominate. Magnetic-pull gauges measure ferrous-substrate coatings (paint on steel) up to about 6 mil. Eddy-current gauges measure non-ferrous (paint on aluminum). Ultrasonic gauges handle paint on plastic and other non-conductive substrates by timing an acoustic echo.

For plastic film, a micrometer (the tool, calibrated in mil or µm) directly measures sheet thickness with about ±0.1 mil resolution. Optical comparators handle PCB copper. All these gauges read in mils, microns, or both — modern digital units switch units with a button.

Common micron-to-mil mistakes

The most common error is conflating mil and mm. A "5 mil coating" is 0.005 in (127 µm) — far thinner than the 5 mm (5000 µm) that the mil/mm confusion implies. The factor between them is 39.37 — a thousand-fold difference, not a unit-error correction.

The second error is the angular-mil confusion described above. A scope adjustment of "1 mil" moves the point of impact 1/6400 of the firing circle. That has nothing to do with paint thickness even though both use the word mil.