In our models it's often described in ounces/sq foot.

Take the area of the wing in square inches and divide that by 144. You now have the area of the model in square feet.
Take the weight of the airplane in ounces.
Divide the weight by the area.

The Fling 2M glider reviewed recently in R/C Report weighed 22.3 ounces. It's wing area was 477 sq.in. which would be 3.3 sq.ft.
22.3 / 3.3 = 6.75 oz/sq.ft.