Mil and millimeters measure thickness. GSM and ounces per square yard measure mass per area. You can convert mil to mm and GSM to oz/yd², but there is no universal GSM-to-mil conversion for tarps.
Many product comparisons put mil, GSM, and ounces in the same table as if they are different scales for the same property. They are not. A useful chart must separate sheet thickness from fabric mass and explain what construction can change.
Mil to millimeter chart
One mil is one thousandth of an inch. Because one inch is exactly 25.4 mm, one mil is exactly 0.0254 mm.
| Mil | Inches | Millimeters | Micrometers |
|---|---|---|---|
| 5 mil | 0.005 in | 0.1270 mm | 127.0 μm |
| 6 mil | 0.006 in | 0.1524 mm | 152.4 μm |
| 8 mil | 0.008 in | 0.2032 mm | 203.2 μm |
| 10 mil | 0.010 in | 0.2540 mm | 254.0 μm |
| 12 mil | 0.012 in | 0.3048 mm | 304.8 μm |
| 14 mil | 0.014 in | 0.3556 mm | 355.6 μm |
| 16 mil | 0.016 in | 0.4064 mm | 406.4 μm |
| 18 mil | 0.018 in | 0.4572 mm | 457.2 μm |
| 20 mil | 0.020 in | 0.5080 mm | 508.0 μm |
| 24 mil | 0.024 in | 0.6096 mm | 609.6 μm |
| 30 mil | 0.030 in | 0.7620 mm | 762.0 μm |
This conversion is exact mathematically. The product’s real thickness can still vary because of manufacturing tolerance, texture, embossed surfaces, woven reinforcement, and where the measurement is taken.
GSM to oz/yd² reference
| GSM | oz/yd² | GSM | oz/yd² |
|---|---|---|---|
| 90 | 2.65 | 300 | 8.85 |
| 140 | 4.13 | 450 | 13.27 |
| 180 | 5.31 | 610 | 17.99 |
| 250 | 7.37 | 680 | 20.06 |
Why GSM cannot be converted directly to mil
Mass, thickness, and area are related through density only for a uniform solid material. A tarp may contain woven fibers, gaps in the weave, multiple coatings, surface texture, scrim reinforcement, hems, and laminated layers. Two tarps can weigh the same per square meter but use different thicknesses and constructions.
Even for two solid sheets at the same GSM, a lower-density material would be thicker than a higher-density material. Without reliable density and construction data, a GSM-to-mil number is an assumption, not a conversion.
Use like-for-like comparisons
Compare mil with mil for sheet thickness. Compare GSM with GSM or oz/yd² for fabric mass. Then compare strength, coating, reinforcement, and finished construction separately.
How tarp thickness is measured
A thickness gauge or micrometer compresses material between two faces. The result depends on contact pressure, foot size, surface texture, reinforcement, and whether the gauge lands on a yarn crossover or a flatter coated area. Responsible specifications identify a test method and tolerance.
A handheld caliper can provide a rough check but is easy to squeeze too hard, angle incorrectly, or place on a raised feature. It should not be treated as a laboratory result.
How to compare two listings
- Confirm that both numbers refer to the finished tarp material.
- Identify the unit and property: thickness, mass per area, or total weight.
- Look for tolerance and test method.
- Check base fabric, coating, reinforcement, seam, and hardware details.
- Compare finished dimensions and total finished weight.
- Treat unsupported adjectives such as “super heavy duty” as marketing, not a measurement.
For coated woven products, a complete listing should make clear whether the quoted mass includes the coating. For vinyl truck tarps, ounces per square yard commonly refers to the finished coated fabric, but the product listing should say so.
Common mistakes
- Reading 10 mil as 10 mm. Ten mil is 0.254 mm.
- Reading oz/yd² as total ounces. It is mass per square yard.
- Converting GSM to mil with a generic online ratio.
- Assuming a thicker sheet is always stronger in tear.
- Comparing cut size to finished size.
- Ignoring the weight added by a very large area.
Exact conversion sources: NIST Unit Conversion and NIST Handbook 44, Appendix C. Use the Tarp Unit Converter for precise calculations.
How this guide was prepared
We separate geometry and unit conversions from product claims, use exact standards where available, and flag specifications that vary by manufacturer. See our editorial policy for the review process.