RV Roof Coating Thickness Conversion Reference: Mils, mm and Inches
Use this reference when an RV roof coating proposal, product data sheet or inspection record expresses thickness in different units. It converts the number; it does not select the right thickness for a roof system or verify what was installed.
A mil is not a millimeter. One mil is one-thousandth of an inch: 0.001 inch, 0.0254 millimeter or 25.4 micrometers. These relationships are exact. See the thickness-measurement table in NIST Handbook 133, Appendix E.
Mils to millimeters and inches
The values below are conversion examples, not recommended application thicknesses. The row you need depends on the number in the actual specification.
| Mils | Millimeters (mm) | Inches (in) | Micrometers (µm) |
|---|---|---|---|
| 1 | 0.0254 | 0.001 | 25.4 |
| 10 | 0.254 | 0.010 | 254 |
| 20 | 0.508 | 0.020 | 508 |
| 30 | 0.762 | 0.030 | 762 |
| 40 | 1.016 | 0.040 | 1016 |
| 60 | 1.524 | 0.060 | 1524 |
| 80 | 2.032 | 0.080 | 2032 |
| 100 | 2.540 | 0.100 | 2540 |
| 120 | 3.048 | 0.120 | 3048 |
| 125 | 3.175 | 0.125 | 3175 |
Convert a value that is not in the table
- Mils to millimeters: multiply by 0.0254.
- Millimeters to mils: divide by 0.0254.
- Mils to inches: divide by 1000.
- Mils to micrometers: multiply by 25.4.
For example, 75 mils × 0.0254 = 1.905 mm. In the opposite direction, 2 mm ÷ 0.0254 is approximately 78.74 mils. Keep the original stated value beside your conversion so that rounding does not hide a difference.
Keep the measurement description attached
Two numbers can have matching units but describe different things. Before comparing them, copy the complete thickness requirement, including the product, layer and measurement condition. A short record is more useful than an isolated number:
- Original entry: the value, unit and exact specification wording.
- What it describes: one coat, a named layer or the complete specified system.
- Condition: wet-film thickness, dry-film thickness or another stated condition.
- Requirement: minimum, nominal, maximum or a range.
- Source: document title, revision and page.
Dry-film thickness concerns the cured coating. Protective-coating specifications can distinguish nominal, minimum and maximum thickness; the largest number is not automatically the right target. Elcometer’s dry-film thickness guide explains these distinctions.
This table changes units only. It does not convert wet-film thickness into dry-film thickness. If one document gives a wet value and another a dry value, obtain the applicable product guidance before treating them as equivalent.
Use a conversion to clarify, not certify
If a proposal says “about 3 mm” and a specification gives a value in mils, ask the contractor to identify the controlling requirement rather than choosing a rounded equivalent yourself. A calculated value does not establish the accuracy of a measurement, the suitability of an instrument or compliance across an entire roof.
Ask how the specified thickness will be documented on the particular substrate and which layers the result includes. Keep that answer with the product documents and completion record. Do not cut, scrape or puncture a roof coating to check a conversion.
Bottom line: convert the units, preserve the original number and compare only entries that describe the same layer and condition. The approved roof-system specification remains the source for the required thickness.