Post Hole Concrete Calculator
Estimate concrete for fence, deck, mailbox, and sign posts. Enter hole diameter and depth, plus how many holes, to get cubic yards and bag counts.
How deep should a post hole be?
| Post type | Typical depth | Typical diameter |
|---|---|---|
| 3–4 ft fence post | 2 ft | 8–10 in |
| 5–6 ft fence post | 2.5–3 ft | 10–12 in |
| Deck post | 3 ft (or below frost line) | 12–14 in |
| Mailbox post | 1.5–2 ft | 8–10 in |
How this concrete post hole calculator works
Each post hole is treated as a cylinder: π × (diameter ÷ 2)² × depth. If you enter the actual post width, the calculator subtracts the square post volume inside the hole, applies your waste allowance, and rounds each bag option up to a whole bag.
Nominal lumber is smaller than its name: a typical 4×4 post is about 3.5×3.5 inches, and a typical 6×6 is about 5.5×5.5 inches. Leave the field blank if you prefer a conservative estimate based on the full hole volume.
Common post hole concrete needs
| Hole | Concrete needed | 80 lb bags |
|---|---|---|
| 10 in diameter × 2 ft deep | 1.09 cu ft before post subtraction | 2 |
| 12 in diameter × 3 ft deep | 2.36 cu ft before post subtraction | 4 |
| 14 in diameter × 3 ft deep | 3.21 cu ft before post subtraction | 6 |
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FAQ
How deep should a fence post hole be?
Fence posts are usually set in holes 2 to 3 feet deep, or roughly one-third of the post length above ground, whichever is greater.
How many bags of concrete per fence post?
A 10-inch diameter hole 2 feet deep has about 1.09 cubic feet before subtracting the post. With a 10% allowance, plan on roughly 2 × 80 lb bags, 3 × 60 lb bags, or 4 × 40 lb bags.
How much concrete for a 6×6 deck post?
A 12-inch diameter hole 3 feet deep has about 2.36 cubic feet before subtracting the post. Enter the actual post width—commonly about 5.5 inches for nominal 6×6 lumber—for a more specific estimate.