Three numbers that are not interchangeable
Most confusion in tarp shopping comes from treating three different quantities as if they measured the same thing. They do not.
| Spec | What it measures | What it cannot tell you |
|---|---|---|
| Mil | Thickness of a sheet, in thousandths of an inch | Nothing about scrim count, coating quality, or tear strength |
| GSM | Mass of fabric per square meter | Nothing about how that mass is distributed between scrim and coating |
| oz/yd² | The same quantity as GSM, in US customary units | Same limits as GSM; 1 oz/yd² ≈ 33.906 GSM |
A 10 mil PE tarp and a 10 oz canvas are not comparable by their headline numbers, because one is a thickness and the other is a mass. The thickness chart works through which unit applies to which product, and the unit converter handles the arithmetic when a listing gives you the wrong one.
What actually decides service life
Fabric mass is a useful proxy, but it is not the whole answer. Two 550 GSM PVC tarps can behave very differently depending on the scrim underneath the coating, whether the coating is laminated or knife-coated, whether the hems are welded or stitched, and how the corners are reinforced. On a cover that is repeatedly deployed and folded, the seams and hardware usually reach end of life before the panel does.
UV exposure is the other variable people underestimate. An untreated PE tarp in direct summer sun can lose meaningful strength in a single season, while a UV-stabilized fabric of the same weight lasts several. If the cover stays up year-round, UV treatment matters more than adding 100 GSM.
Breathability is a real trade-off, not a lesser option
A fully waterproof cover over firewood, hay, or a stored engine traps moisture underneath and can do more damage than the rain would have. Canvas and mesh exist for those jobs. Sealing something wet is not the same as protecting it.
Where to start
New to the categories, start with Types of Tarpaulin Materials for the overview, then PVC vs PE for the comparison that covers most buying decisions. If you already know you need coated fabric, How to Choose PVC Tarpaulin is the specification checklist. For fabric weight specifically, read the GSM guide before comparing listings. Not sure which category fits at all — the Material Selector asks about the job and narrows it for you.