Post Depth
42 in
Hole Dia
12 in
80 lb Bags
74
Recommended Burial Depth
42
inches
Hole Diameter
12 in
Posts
16
Concrete / Post
2.75 cu ft
80 lb Bags
74
Inputs
Result
This is a typical example where frost line drives a deeper hole than the simple one-third rule alone.
Inputs
Result
One-third of 48 inches is 16 inches, but the 24-inch minimum applies. Clay soil holds well so no extra depth is needed beyond the minimum.
A common baseline is about one-third of the fence height in the ground, but frost line, wind exposure, and loose soils often require deeper holes. In cold climates, the bottom should usually extend below the frost line.
A practical rule is roughly three times the post width, with wider holes often used for taller fences or unstable soils. Hole diameter affects concrete volume just as much as depth does.
Most 4x4 posts in standard 10-inch diameter holes need 1 to 2 bags of 50-lb premixed concrete depending on hole depth. Deeper holes or 6x6 posts in wider holes can require 3 to 4 bags each.
| Hole Depth | Hole Diameter | Approx. Bags (50 lb) |
|---|---|---|
| 24 in | 10 in | 1 |
| 36 in | 10 in | 1.5 to 2 |
| 36 in | 12 in | 2 to 2.5 |
| 42 in | 12 in | 3 |
Yes. In cold climates, post holes should extend below the frost line to prevent frost heave from pushing posts upward over freeze-thaw cycles. This often means 36 to 48 inches deep even for shorter fences.
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Last Updated: Mar 11, 2026
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