Bags per hole
| Hole width | Depth | Cubic feet (+10%) | 80 lb bags (exact) | 80 lb (rounded up) | 60 lb | 40 lb | 80 lb net of post |
|---|---|---|---|---|---|---|---|
| 12 in | 30 in | 2.16 | 3.6 | 4 | 5 | 8 | 2.7 |
| 12 in | 36 in | 2.59 | 4.3 | 5 | 6 | 9 | 3.3 |
| 12 in | 42 in | 3.02 | 5.0 | 6 | 7 | 11 | 3.8 |
| 14 in | 42 in | 4.12 | 6.9 | 7 | 10 | 14 | 5.6 |
| 16 in | 48 in | 6.14 | 10.2 | 11 | 14 | 21 | 8.8 |
Hole volume = π × (diameter ÷ 2)² × depth, with diameter and depth in feet
Bags = volume ÷ bag yield (80 lb = 0.60 ft³), rounded up per hole, +10% extra
Total bags for a run of posts
| Posts | 80 lb bags (exact) | 80 lb bags to buy | Bag cost | Cubic yards |
|---|---|---|---|---|
| 1 | 5.0 | 6 | $39 | 0.11 |
| 4 | 20.1 | 21 | $136 | 0.45 |
| 10 | 50.3 | 51 | $332 | 1.12 |
| 20 | 100.7 | 101 | $656 | 2.24 |
| 50 | 251.7 | 252 | $1,638 | 5.59 |
Calculated for the 12 in × 42 in hole. Holes are rounded up to whole bags when buying. For 50 or more posts ready-mix or a mix-your-own approach can be cheaper: see bags vs ready-mix.
Net-of-post figures subtract the post’s own volume (5.5 × 5.5 in) and use the same 10% extra; fast-setting mixes may yield slightly less per bag.
Related
Also see 4x4 post , mailbox post , chain link post, the fence post chart and the concrete calculator (choose “Post holes”).