Fence Post Concrete Calculator
Enter your hole size, post size, and number of posts. You get the bag count per post and the total for the job — instantly, as you type.
Quick reference: bags per post
| Post | Hole (dia × depth) | 50 lb fast-set bags* |
|---|---|---|
| 4x4 | 10" × 24" | 2 |
| 4x4 | 12" × 24" | 3 |
| 4x4 | 12" × 30" | 4 |
| 4x4 | 12" × 36" | 5 |
| 6x6 | 14" × 24" | 4 |
| 6x6 | 16" × 30" | 7 |
| 6x6 | 18" × 36" | 11 |
*Assumes a 6" gravel base under the post and concrete filling the rest of the hole — the standard install. Skipping gravel? Add roughly one bag per post.
How the math works
Concrete fills the hole around the post, above the gravel base, so the formula is hole volume minus post volume over the concrete depth: π × (hole radius)² × fill depth − post width² × fill depth, where fill depth = hole depth − gravel base. A 50 lb bag of fast-setting mix yields 0.375 cubic feet. Bags round up per post — a partial bag left in the truck beats a partially set post.
Rule of thumb: hole diameter = 3× post width, hole depth = ⅓ of total post length (minimum 24"), and always below your local frost line.
Common questions
How many bags of concrete do I need per fence post?
Most 4x4 fence posts set in a 12-inch wide, 24-inch deep hole need about 3 bags of 50 lb fast-setting concrete. Larger 6x6 posts or deeper holes need 3–4 bags. Use the calculator above with your exact hole size for a precise count.
How deep should a fence post hole be?
Bury one-third of the total post length, with a minimum depth of 24 inches for a 6-foot fence. In cold climates, dig below the frost line so freeze–thaw cycles can't heave the post.
How wide should the hole be?
Three times the post width. A 4x4 post (3.5 in actual) needs a hole about 10–12 inches in diameter.
Should I use fast-setting concrete for fence posts?
Yes for most fences. Fast-setting mix is poured dry into the hole and watered — no mixing — and supports the post in 20–40 minutes. Standard mix is fine too but needs bracing for 24+ hours.