How to calculate concrete for fence posts
Hole area = π × (diameter ÷ 2)²
Example: a 4×4 post (3.5 in actual) in a 10 inch hole, 30 inches deep with 6 inches of gravel at the bottom, needs about 0.92 cubic feet of concrete — roughly 2.5 bags of 50 lb fast-setting mix or 1.5 bags of 80 lb standard concrete per post.
How big should the hole be?
| Post | Hole diameter | Typical depth |
|---|---|---|
| 4×4 (3.5 in) | 10–12 in | 2–3 ft |
| 6×6 (5.5 in) | 14–16 in | 3 ft or more |
| Round 3 in metal post | 8–10 in | 2–3 ft |
Common rules of thumb: the hole is about 3 times the post width, and about one-third to one-half of the post's above-ground height deep (at least 2 ft). In cold climates, dig below the frost line.
Fast-setting or standard?
Fast-setting mix is poured dry into the hole and then watered — no mixing, and posts can bear load in a few hours. Standard concrete is stronger and cheaper per cubic foot but must be mixed first.
Frequently asked questions
How many bags of concrete per fence post?
Typically 2–3 bags of 50 lb fast-setting mix for a 4×4 post in a 10 inch hole about 2½ ft deep.
How deep should a fence post hole be?
About one-third to one-half of the post's above-ground height, at least 2 ft, and below the frost line in cold areas.
Should I put gravel at the bottom of a post hole?
Yes — 4–6 inches of gravel under the post helps water drain away from the end of the post.
How much concrete does an 80 lb bag make?
About 0.6 cubic feet; a 50 lb bag of fast-setting mix makes about 0.375 cubic feet.
Related calculators
These figures are estimates for planning only. Material weights and coverage vary by supplier, moisture and compaction. Always confirm quantities and prices with your supplier before ordering. See our terms & disclaimer.