Fence Post Concrete Calculator
Bags per post and for the whole fence line. Defaults are the standard 4×4 post in a 10" × 24" hole, the "2½ bags per post" setup. Posts are rounded to half-bags because you batch each hole separately.
Hole volume minus post volume, per post. Add 4–6" of gravel below each post for drainage (not included in concrete depth). Fast-setting 50 lb yield: 0.375 ft³.
Cheat sheet: bags per post
| Setup | Hole | 50 lb fast-setting | 80 lb mix |
|---|---|---|---|
| 4×4 post, 4 ft picket fence | 10" × 18" | 2 bags | 1 bag |
| 4×4 post, 6 ft privacy fence | 10" × 24" | 2½ bags | 1½ bags |
| 4×4 post, windy site / soft soil | 12" × 30" | 4 bags | 2½ bags |
| 6×6 gate post | 12" × 30" | 3½ bags | 2½ bags |
| Mailbox post | 8" × 20" | 1½ bags | 1 bag |
Yields for every bag size, including how many bags ride a pallet, are in the concrete bag yields guide. If a fence run lands near a yard of mix, use the yard converter. Fast-setting dry-pour steps (and when not to dry-pour) are covered in dry pour concrete: what actually works.
Two posts that aren't fences
The math is identical for anything you set in a hole. Two come up constantly.
Mailbox post
A 4×4 mailbox post in an 8" × 20" hole is about 0.44 ft³ of concrete once you subtract the post, so 1.5 × 50 lb fast-setting bag does it (buy 2 so you are not short). Brace it plumb, dry-pour the fast-setting mix, add about a gallon of water, and crown the top so rain runs off the wood instead of pooling against it. A single 60 or 80 lb bag also works if that is what is on the shelf.
In-ground basketball hoop
Bigger than people expect. A typical in-ground pole or anchor kit calls for a hole around 18" across and 32" deep, which is about 4.71 ft³: roughly 13 × 50 lb bags in one pour. Set the anchor bolts or pole sleeve while the concrete is wet and check it plumb twice. Here is the honest part: your hoop's manual prints the exact hole size and bag count it was engineered for, and a backboard is a real wind sail on a lever arm. Follow that spec, not a generic number, and use this calculator only to sanity-check what the manual tells you to buy.
Setting posts that stay plumb
- Dig 3× the post width, to one-third of fence height (min 24", below frost line where relevant).
- Drop 4–6" of gravel in the bottom; tamp it. The post should never sit in a concrete cup that traps water.
- Set the post, brace with two diagonal 2×4s, check plumb on two faces.
- Fast-setting mix: pour dry, add ~1 gallon of water per 50 lb bag, done in 30 minutes. Standard mix: wet-mix, shovel in, re-check plumb.
- Slope the concrete top away from the post, and let gate posts cure 24 hours before hanging weight.
FAQ
How many bags of concrete per fence post?
The classic residential setup (a 4×4 post in a 10-inch hole, 24 inches deep) takes about 0.9 ft³ of concrete: 2½ × 50 lb bags of fast-setting mix, or 1½ × 80 lb bags per post. Bigger 6×6 gate posts in a 12-inch hole, 30 inches deep, run 3–4 × 50 lb bags each.
How deep should fence posts be set?
Rule of thumb: one-third of the above-ground fence height, minimum 24 inches, so a 6 ft privacy fence wants posts about 2 ft deep (plus 4–6 inches of gravel below the post for drainage, which the calculator’s depth shouldn’t include). In frost country, get below the local frost line or the fence will heave.
Can I pour fast-setting concrete dry into the hole?
Yes, that’s exactly what fast-setting mix is designed for: set the post, brace it plumb, pour the dry mix around it, then add about a gallon of water per 50 lb bag and let it soak in. It sets in 20–40 minutes and you can hang fence panels in about 4 hours. Standard (non-fast-setting) mix should be wet-mixed first. For slabs and the limits of the method, see dry pour concrete.
Concrete, gravel, or foam for fence posts?
Concrete is strongest for wind-loaded privacy fences and gates. Well-compacted gravel drains better (posts often last longer) and suits open fences in firm soil. Expanding foam is fast and light but weaker in poor soil and pricier per post. Whatever you use, crown the top so water sheds away from the wood.