How the post depth calculator works
The one-third rule, with a frost floor
Wood-post convention: bury one-third of the above-ground height — a 6 ft fence keeps 5.5 ft exposed with ~2.5 ft in the ground. Then the floor: never shallower than your frost line (36–48 inches up north), because frost heave lifts shallow posts out of the ground one winter at a time. The calculator applies both and widens holes for wind exposure — the bell shape at the bottom (a foot wider than the hole) resists uplift.
Hole diameter and concrete
Standard: hole diameter = 3× post width (10–12 inches for a 4×4). Concrete per hole: 10-inch × 36-inch hole ≈ 2.3 bags of 60-lb mix; the calculator totals your post count. Dry-set (pour bag, add water) works in most soils; wet-mix for soft ground.
Details that keep posts plumb
- Crown the concrete — slope the top away from the post so water sheds instead of pooling at the collar.
- Never set wood directly on soil — gravel base 2–4 inches, or posts rated ground-contact (treated .40 CCA or better).
- Brace and check plumb in two axes before the concrete takes — posts never re-plumb themselves.