| Hole | Post | Cu ft per post | 50 lb bags/post |
|---|---|---|---|
| 8″ × 24″ | 3.5″ | 0.53 | 1.41 |
| 10″ × 24″ | 3.5″ | 0.92 | 2.46 |
| 10″ × 30″ | 3.5″ | 1.15 | 3.07 |
| 12″ × 36″ | 3.5″ | 2.10 | 5.60 |
| 12″ × 36″ | 5.5″ | 1.73 | 4.60 |
How the bag count is found
The concrete needed is the hole volume minus the part the post fills. A 10-inch hole 24 inches deep is about 1.09 cu ft; a 4 × 4 post (3.5 × 3.5 in actual) takes up 0.17 cu ft, leaving 0.92 cu ft, which is about 2.5 bags of 50 lb mix (0.375 cu ft each).
Hole size
A common guideline is a hole about three times the post width and a depth of about one-third of the post's total length, plus a few inches of gravel at the bottom for drainage. Local frost depth and fence height may require deeper holes; check local codes.
Frequently asked questions
How big should a fence post hole be?
A common rule is about 3× the post width wide and 1/3 to 1/2 of the above-ground height deep; follow local frost-depth guidance.
Do I need to mix fast-setting concrete?
Fast-setting mixes are usually poured dry into the hole and then soaked with water; follow the bag instructions.
How long before I hang the fence?
Fast-setting mixes are typically ready in about 4 hours; regular mix needs longer.