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 |
|---|---|---|---|---|---|---|---|
| 8 in | 24 in | 0.77 | 1.3 | 2 | 2 | 3 | 1.0 |
| 10 in | 30 in | 1.50 | 2.5 | 3 | 4 | 5 | 2.2 |
| 10 in | 36 in | 1.80 | 3.0 | 3 | 4 | 6 | 2.6 |
| 12 in | 36 in | 2.59 | 4.3 | 5 | 6 | 9 | 3.9 |
| 12 in | 42 in | 3.02 | 5.0 | 6 | 7 | 11 | 4.5 |
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 | 2.5 | 3 | $20 | 0.06 |
| 4 | 10.0 | 10 | $65 | 0.22 |
| 10 | 25.0 | 25 | $162 | 0.56 |
| 20 | 50.0 | 50 | $325 | 1.11 |
| 50 | 125.0 | 125 | $812 | 2.78 |
Calculated for the 10 in × 30 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 (3.5 × 3.5 in) and use the same 10% extra; fast-setting mixes may yield slightly less per bag.
Related
Also see 6x6 post , mailbox post, the fence post chart and the concrete calculator (choose “Post holes”).