GSM and shade percentage explained

Grams per square meter — the weight of the fabric. That is all it directly measures, and it is useful because of what weight correlates with rather than what it is.

Heavier shade fabric generally means a denser weave, thicker monofilaments, or both. In practice that produces three things you care about: more tear strength, more resistance to stretch, and a longer service life.

The stretch resistance is the one that matters most, and it is the least obvious. Shade sails fail through tension loss: fabric stretches, the sail goes slack, a slack sail flogs in wind, and flogging works the corner rings and the stitching beside them until something tears. Heavier fabric stretches less and later, which means fewer re-tensions, less time spent slack, and a longer life.

It does not directly measure shade, which is the confusion this page exists to clear up. A denser weave does block more light — so the two are correlated — but the relationship flattens out quickly, and above roughly 190-200 GSM extra weight is buying durability rather than shade.

These get used interchangeably in listings and they are not quite the same thing, which is worth untangling.

UV block percentage is how much ultraviolet radiation the fabric stops. It is the figure that matters for skin protection and for how fast anything underneath fades, and it is what the sails we track publish: 90% to 95%.

Shade percentage is how much visible light the weave stops — closer to how dark it feels underneath. It is usually similar to the UV figure and not identical, because the fabric's UV stabilizers and pigments affect the two differently.

What you actually feel is a third thing, and it is worth setting expectations about. A breathable mesh at 95% UV block does not feel like standing in building shade: some light comes through, the shade is bright rather than dim, and the fabric color tints it. That brightness is the point of a mesh — it is also why a sealed waterproof panel can feel cooler in terms of light and warmer in terms of trapped air.

Above roughly 90%, the practical difference is small. Below it, you notice. So the useful rule is to treat 90% as a floor rather than treating 95% as meaningfully better than 93%.

Color matters too and no manufacturer quantifies it. Darker fabric generally blocks more light and absorbs more heat; lighter fabric passes more light and stays cooler to the touch. We have not measured this and will not publish a figure for it.

Based on the published range rather than an invented threshold.

Worth knowing, because treating GSM as a single quality score will mislead you in two specific ways.

It does not tell you whether the fabric is virgin or recycled. ColourTree specifies 100% virgin non-recycled HDPE; most manufacturers say nothing. Recycled stock has been through a heat cycle and carries a less predictable additive package, including the UV stabilizers that determine degradation rate. Two 190 GSM sails, one virgin and one recycled, weigh the same and will not age the same.

It does not tell you about the edge. A 260 GSM sail with a webbing edge and a 260 GSM sail with a galvanized cable through the perimeter behave differently under load: the webbing edge distorts and puts a wave into the fabric, and the cable edge stays straight and spreads corner tension along the perimeter. Edge construction is a separate specification and it matters as much as weight on a long span.

And it tells you nothing about wind, because no shade sail manufacturer publishes a wind rating at all. Heavier fabric helps indirectly — it holds tension longer, and a taut sail spills wind where a slack one flogs — but there is no figure, for any weight, from anyone. Best shade sails for high winds covers what actually helps.