NUMBER 1: Check an actual bag and see how many cubic feet it is supposed to cover. I have looked at many sites on web and got several answers. Most frequent was 2/3 of a cubic foot per 80 pound bag, or say 0.67 cf per 80 lb bag.

**The 0.67' below is not related to one above. This 0.67' is 8" converted to feet!

Area = 0.67'/ 2 Square the result x 3.14 = Area in square feet

Area in sq ft x 2.0' (depth)= one sonotube volume in cubic ft

one sonotube volume x 9 = cu ft all sonotubes

I get 9.47 bags, say 10 bags plus one for waste.

But you have to check my math and check on what they claim a bag will do in volume. Usually the big box stores will let you return unopened bags. don't run short- very frustrating in the middle of pouring.

**The 0.67' below is not related to one above. This 0.67' is 8" converted to feet!

Area = 0.67'/ 2 Square the result x 3.14 = Area in square feet

Area in sq ft x 2.0' (depth)= one sonotube volume in cubic ft

one sonotube volume x 9 = cu ft all sonotubes

I get 9.47 bags, say 10 bags plus one for waste.

But you have to check my math and check on what they claim a bag will do in volume. Usually the big box stores will let you return unopened bags. don't run short- very frustrating in the middle of pouring.