Mils and Microns
Coating thickness is expressed in mils (thousandths of an inch) or microns (millionths of a meter). One mil equals 25.4 microns. Knowing both prevents costly specification errors.
Mils and microns are the two units most often used to express the thickness of coatings, paint films and surface profiles. A mil is one thousandth of an inch (0.001 in). It is used mainly in the United States. A micron, more correctly a micrometer (symbol µm), is one millionth of a meter (0.001 mm). It is used in most other countries and in ISO standards. Coating specifications, data sheets, inspection reports and gauge readings use one or both, so applicators and inspectors need to convert between them accurately.
The conversion
1 mil = 25.4 µm
1 µm ≈ 0.03937 mil
Useful rules of thumb:
- Multiply mils by 25.4 to get microns.
- Divide microns by 25.4 to get mils.
- For quick estimates, 1 mil is about 25 µm, and 40 µm is about 1.6 mils.
| Mils | Microns (µm) |
|---|---|
| 1 | 25.4 |
| 2 | 50.8 |
| 3 | 76.2 |
| 4 | 101.6 |
| 5 | 127 |
| 8 | 203.2 |
| 10 | 254 |
| 20 | 508 |
| 40 | 1,016 |
| 60 | 1,524 |
| 125 | 3,175 |
A note on “mil” versus “millimeter”
In some countries, “mil” is used informally for millimeter. This causes confusion. A millimeter equals about 39.4 mils. Specifications should always state units unambiguously, for example “mils (0.001 in)” or “µm.”
Where the units are used
Dry film thickness
Dry film thickness (DFT) is the thickness of the cured coating, and it is the value specifications usually control. Gauges display mils, microns or both (see dry film thickness measurement). Thin primers may be specified at a few mils; high-build linings and elastomers may be tens or hundreds of mils.
Wet film thickness
Wet film thickness (WFT) is measured during application with a notched gauge. It relates to DFT through volume solids:
DFT = WFT × volume solids (as a decimal)
For example, a coating with 50 percent volume solids applied at 8 mils wet gives about 4 mils dry. See wet film thickness measurement and volume solids and coverage.
Surface profile
Blast profiles are also expressed in mils or microns, commonly between about 1 and 5 mils (25 to 125 µm) for steel coatings (see surface profile and surface profile measurement).
Thick-film systems
Spray polyurea, linings and flooring are often specified in tens of mils, for example 40, 60 or 125 mils, or in millimeters outside the United States (1 mm is about 39.4 mils).
Coverage and thickness
Theoretical coverage depends on thickness. In US units, one US gallon of a 100 percent solids coating covers about 1,604 square feet at 1 mil. In metric units, one liter of a 100 percent solids coating covers 1,000 square meters at 1 µm, or 10 square meters at 100 µm. Actual coverage is lower because of losses, profile and overspray.
Specification practice
Specifications typically state:
- Minimum and maximum DFT per coat or for the system.
- Units clearly identified.
- The measurement standard for acceptance, such as SSPC-PA 2 or ISO 19840 (see SSPC-PA 2 thickness acceptance).
When converting a specification, keep the original value and round sensibly. Converting 4 mils to 101.6 µm and then rounding to 100 µm, for example, changes the requirement slightly. Projects should agree on which unit governs.
Gauge settings
Most modern gauges can display either unit. Before inspection:
- Confirm the gauge unit setting.
- Record units in reports.
- Verify the gauge against reference standards in the same unit (see coating thickness gauge calibration, verification and adjustment).
Common mistakes
| Mistake | Consequence |
|---|---|
| Confusing mils with millimeters | Thickness off by a factor of about 39 |
| Reading the gauge in the wrong unit | Rejected or accepted work in error |
| Forgetting volume solids when setting WFT | Under- or over-thick films |
| Rounding during double conversion | Small, accumulating differences |
| Mixing units in one report | Confusion in acceptance |
Other related units
- Thou is a British term for a mil.
- Gauge for sheet metal and plastic films is a separate system and does not convert directly to mils.
- Millimeters are used for thick coatings and flooring.
- Nanometers (nm) are used for very thin films and pretreatments (see nanocoatings).
In practice
In practice, it helps to state the unit on every document, from the specification to the inspection report. Many problems start when a data sheet in microns is read by a crew used to mils, or the reverse. Gauges can usually be switched between units, so inspectors should confirm the setting before measuring. A simple conversion table on the job sheet reduces mistakes.
Key terms
- Mil. One thousandth of an inch.
- Micron. One micrometer, one thousandth of a millimeter.
- WFT. Wet film thickness, measured right after application.
- DFT. Dry film thickness, measured after the coating has cured.
- Volume solids. The percentage of a coating that remains as dry film.
- Coverage rate. Area covered per volume at a given thickness.
Frequently asked questions
How many microns is 10 mils?
254 µm.
How many mils is 200 microns?
About 7.9 mils.
Is a mil the same as a millimeter?
No. A mil is 0.0254 mm.
Related entries
- Dry film thickness measurement
- Wet film thickness measurement
- Volume solids and coverage
- Surface profile
- SSPC-PA 2 thickness acceptance (new)
- Concrete surface profile (CSP) (new)
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
