Using 2 cubic foot bags:

One 2 cubic foot bag covers…

DEPTH	Area Covered: 8’x8′ (64 sq. ft.)	Area Covered: 25’x4′ (100 sq. ft.)	Area Covered: 12’x12′ (144 sq. ft.)
2″ = 12 sq. ft.	5.3 bags needed	8.3 bags needed	12 bags needed
3″ = 8 sq. ft.	8 bags needed	12.5 bags needed	18 bags needed
4″ = 6 sq. ft.	10.6 bags needed	16.6 bags needed	24 bags needed