Post Hole Calculator
Calculate the concrete needed per post hole and across your whole project. Enter the hole diameter and depth, the post size, and the number of holes to get bags or volume, with the post displacement already subtracted. Works for fence posts, deck footings, and mailbox posts.
Post Hole Diagram
How the Formula Works
Calculate hole volume (cylinder)
Hole Volume = π × r² × Depth r = half the diameter; a 10″ diameter × 3 ft deep hole = π × (5/12)² × 3 = 1.636 ft³
Calculate gravel base volume
Gravel Volume = π × r² × Gravel Depth Subtract from total hole to get concrete zone; 6″ gravel base in 10″ hole = 0.273 ft³
Subtract post displacement
Post Displacement = Post Width² × (Depth − Gravel Depth) A 4×4 post (3.5″ actual = 0.292 ft side) displaces 0.292² × 2.5 ft ≈ 0.213 ft³ of concrete space; using nominal for conservative estimate
Calculate concrete volume per hole
Concrete = Hole Volume − Gravel Volume − Post Displacement Apply waste factor: Concrete × (1 + Waste%); example: 1.636 − 0.273 − 0.213 = 1.15 ft³ net, with 10% = 1.27 ft³
Calculate bags required
Bags per Hole = ⌈Concrete per Hole ÷ Yield per Bag⌉ 80 lb bag = 0.60 ft³ → 1.27 ÷ 0.60 = 2.11 → 3 bags per hole; total = bags per hole × number of holes
Frequently Asked Questions
As a rule of thumb, most 8-ft fence posts with a 4×4 in a 10″ × 3 ft hole need 2–3 bags of 80 lb concrete.
Bury 1/3 of the total post length. An 8-ft post needs at least 2.5 ft below grade. Add 6 inches below the frost line in cold climates.
At minimum 3× the post width. A 4×4 post needs a 12″ hole; a 6×6 post needs an 18″ hole. Go wider in sandy or loose soil.
Yes — 4–6 inches of crushed gravel improves drainage and significantly extends post life by preventing standing water at the base.
10″ × 2.5 ft deep: 1–2 bags of 80 lb. 10″ × 3 ft deep: 2–3 bags. 12″ × 3.5 ft: 3–4 bags.
Fast-setting concrete (Quikrete Fast-Setting) can be poured dry and sets in 20–40 minutes. Standard mix also works well and is slightly less expensive.
Fast-setting mix: 20–40 minutes. Standard mix: 24–48 hours before attaching fence panels. Cold weather slows the cure significantly.
It varies by location. Zone 6: 18–24″, Zone 5: 30–36″, Zone 4: 36–48″. Always check local building code — footings must extend below the frost line.