It's cubic inches, not square feet, and one can will give you just
about enough to seal around a door with a one-inch gap. I would say
that is about 1200 cubic inches, or 2/3 of a cubic foot.
Note that there are different versions. Some expand more than others,
on purpose. The minimally-expanding type is for applications where
things might move, like a window or door frame.
Using basic math volume formulas and the detail that a 16 ounce can of Great
Stuff will make a 1/2" diameter bead, by 232' long, the volume of foam in that
bead is ~0.31 cubic feet or an equivalent block of ~12" x12" x 4".
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