Two tarps can both be called "heavy duty" and be completely different products. The numbers on a listing are the way to tell them apart, but they measure different things, and sellers don't always use them the same way.
Ounces per square yard (oz)
The most common figure on vinyl and canvas tarps is weight: how many ounces one square yard of the finished fabric weighs. More weight usually means a heavier base fabric, more coating, or both, and so a tougher tarp. For PVC-coated polyester, 13 to 14 oz is medium duty, 18 oz is the standard heavy-duty weight for flatbed and long-term tarps, and 22 oz and up is for the heaviest work.
Grams per square meter (GSM)
GSM is the same idea in metric units. One ounce per square yard is about 34 GSM, so 18 oz is about 610 GSM. Manufacturers outside the US usually quote GSM, and it's a handy cross-check: a seller listing an "18 oz" tarp at 500 GSM is describing something closer to 15 oz.
Mil (thickness)
A mil is a thousandth of an inch. Thickness is the usual spec for poly tarps and clear vinyl, and many vinyl listings give it too. It does not convert directly from weight. Tarps Now, for example, lists its 14 oz vinyl at about 18 mil and its 22 oz vinyl at about 28 mil; an 18 oz clear vinyl tarp is listed at about 20 mil. The base fabric and the coating both affect thickness, so read weight and thickness together, and don't compare a poly tarp's mil to a vinyl tarp's ounces.
Denier and thread count
Under the coating of a vinyl tarp is a woven polyester "scrim." Denier is the weight of the yarn (higher is thicker yarn), and thread count is how many yarns per inch in each direction. A 1000-denier, 20 x 20 scrim is stronger than a 600-denier, 18 x 17 one even if the coating hides the difference. Not every seller lists these, but when they do, they're the best clue to tear strength.
Cut size and finished size
Many tarps, especially poly, are sold by cut size: the size of the sheet before the edges are folded and hemmed. The finished tarp can be 3 to 5 percent smaller. A "10 x 12" may measure closer to 9.5 x 11.5 ft. Flatbed tarps are usually sold at finished size, but if every inch matters, ask.
Grommets and D-rings
The tie-down points matter as much as the fabric. Look at the spacing (every 2 to 3 ft is typical), the material (brass or stainless don't rust; aluminum is lighter and softer), and how the edge is reinforced. Flatbed tarps use D-rings stitched to webbing because they take far more load than a grommet punched through fabric.
What to compare
- Vinyl: weight (oz or GSM) first, then thickness and denier.
- Poly: thickness in mil and the weave count; weight if given.
- Canvas: finished weight and the treatment.
- Everything: finished size, grommet spacing and material.
Doing the math
A few conversions let you check any listing yourself.
- Weight to GSM: multiply ounces per square yard by about 33.9. 18 oz × 33.9 ≈ 610 GSM.
- Mil to millimeters: 1 mil = 0.0254 mm, so 20 mil is about 0.5 mm.
- How much a tarp weighs: square feet ÷ 9 = square yards; square yards × ounces per square yard ÷ 16 = pounds of fabric. A 20 x 30 ft tarp is 600 sq ft, or about 66.7 sq yd. At 18 oz that's 1,200 oz, or 75 lb of fabric, before hems, grommets or D-rings.
The last one is a good honesty check. If a listing claims "18 oz" but the shipping weight is far below what the math says, ask why.
Weight versus strength. Fabric weight tells you how much material there is, not how it's built. Two 18 oz tarps can differ in tear strength because of the yarn (denier), the weave density and the coating. When a supplier publishes tensile or tear strength, usually in pounds per inch from a standard test, that's the better number to compare.
Sources
- Tarps Now, 14 oz vinyl tarps (thickness listed)
- Tarps Now, 22 oz super heavy duty vinyl tarps
- Tarps Now, 18 oz heavy duty vinyl tarp
- Manufacturer specification sheets for PVC-coated polyester (GSM and oz pairs), pvctarpaulin.com
Written by the TrueTarps team. Questions about your job? Contact us or request a custom tarp.