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 | 1.2 |
| 8 in | 30 in | 0.96 | 1.6 | 2 | 3 | 4 | 1.5 |
| 8 in | 36 in | 1.15 | 1.9 | 2 | 3 | 4 | 1.8 |
| 10 in | 36 in | 1.80 | 3.0 | 3 | 4 | 6 | 2.9 |
| 12 in | 42 in | 3.02 | 5.0 | 6 | 7 | 11 | 4.9 |
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 | 1.6 | 2 | $13 | 0.04 |
| 4 | 6.4 | 7 | $46 | 0.14 |
| 10 | 16.0 | 16 | $104 | 0.36 |
| 20 | 32.0 | 32 | $208 | 0.71 |
| 50 | 80.0 | 80 | $520 | 1.78 |
Calculated for the 8 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 (2.375 × 2.375 in) and use the same 10% extra; fast-setting mixes may yield slightly less per bag.
Related
Also see mailbox post , deck post , 6x6 post, the fence post chart and the concrete calculator (choose “Post holes”).