The correct way to quote the weight of cloth is in it’s loomstate, meaning, before the material is dyed and proofed, because dying and waterproofing adds to the weight of the material. Darker coloured  dyes weigh more than paler dyes. In fact, only after the base cloth has been proofed, is it called canvas

A square yard of base cloth weighing 12 ounces (12oz) will come up to about 14oz after it has been dyed and proofed.

In metric terms 410 grams per square metre loomstate cloth equals about 480g/m2 after dyeing and proofing is completed.

Be aware of some stores quoting the heavier weight to make it sound better.

When a manufacturer claims to make Swags from 100% cotton canvas, sometimes referred to as swag canvas, don’t be impressed. It sounds good, but an all-cotton canvas is only a budget canvas, it is weak and cheap to make. The thread in the weft direction is only a single thread and tears easily. It often leaks when you rub it, just like the old tents did some many years ago.

Good canvas should be a blend of cotton and polyester threads for superior strength and water tightness.

Also note, that good polycotton/canvas does not need to be soaked in water before use, in fact, it is almost impossible to make it wet because the water will bead and run off. If it were necessary to wet canvas to make it water tight, then it was not water tight in the first place.