Cotton duck canvas for artists’ canvases is classified either as "single fill" (referred to by weight in ounces per square yard) or as "double fill" (which is known by a number system from #1 to #13 - the greater the number, the lighter the canvas). The difference between these two types of canvas is the way they are woven. The strands that run the length of the bolt of fabric are called the warp, and the strands that run perpendicular to the bolt are called the weft. Single-fill canvas uses a single weft strand woven over and under single strands of the warp. Double-fill canvas instead uses two or more threads spun together for the weft and, for #8 and #6 fabrics, also on the warp. The spun strands in the fabric create a tighter weave, making double-fill canvas denser and less permeable to liquid, which will absorb less sizing, gesso, and paint.

For numbered cotton ducks, smaller numbers mean a thicker and heavier canvas.

Upper Canada Stretchers currently carries cotton duck canvas in 7 oz (light weight), #13 (medium weight), #12 (heavy weight), and #10 (extra heavy weight).

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