Take LENGTH x WIDTH in feet, then divide it by how many inches DEEP that you want to go:
1" divide by 324
2" divide by 162
3" divide by 108
4" divide by 81
5" divide by 64
6" divide by 54
7" divide by 46
8" divide by 40
9" divide by 36
10" divide by 32
11" divide by 30
12" divide by 27
100' x 100' = 10,000 square feet
Going 3" deep, 10,000 divided by 108 = 92.59 yards of material.