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 | 24 in | 1.20 | 2.0 | 2 | 3 | 4 | 1.7 |
| 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 |
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 | 2.0 | 2 | $13 | 0.04 |
| 4 | 8.0 | 8 | $52 | 0.18 |
| 10 | 20.0 | 20 | $130 | 0.44 |
| 20 | 40.0 | 40 | $260 | 0.89 |
| 50 | 100.0 | 100 | $650 | 2.22 |
Calculated for the 10 in × 24 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 , chain link post , 4x4 post, the fence post chart and the concrete calculator (choose “Post holes”).