Fences, decks, mailboxes, sign posts

Post hole concrete calculator

Enter hole diameter and depth, set the post size to deduct, and get cubic feet, cubic yards, and bag counts — per hole and for the whole project.

Quick answer: Hole volume is π × radius² × depth. A 12 in diameter × 36 in deep hole holds π × 0.5² × 3 = 2.36 ft³ of concrete. At 0.60 ft³ per 80 lb bag: ceil(2.36 / 0.60) = 4 bags. Add 10% because concrete sinks into loose soil.

Hole dimensions

Rule of thumb: 3× the post width
1/3 of post length, or below frost line
3.5 in for a 4×4 post. Enter 0 to skip.
Concrete sinks into loose soil — 10% is the safe default
Concrete needed
Per hole (net of post)2.10 ft³
1 hole — total2.10 ft³
Cubic yards0.078
Cubic meters0.059
With 10% overage2.31 ft³
Bags (rounded up, includes overage)
80 lb bags4
60 lb bags6
50 lb bags7
40 lb bags8
90 lb bags4
Bag yields are manufacturer-rated: an 80 lb bag pours 0.60 ft³, so bags = ft³ ÷ yield, rounded up. Under 2 yards, bags beat ready-mix — see the concrete bag calculator for the breakeven.

Ballpark, not gospel. Hole walls are never perfect cylinders — irregular sides and loose soil can add 10–20% beyond the math. Bell out the bottom of the hole in frost country for uplift resistance.

Sizing rules that keep posts standing

  • Width: hole diameter ≈ 3× the post width. A 4×4 (3.5 in) wants a 10–12 in hole.
  • Depth: one-third to one-half of the post's above-ground length — or deeper than the frost line, whichever is deeper.
  • Frost line: 12 in in the deep South, up to 48 in in the northern states. Consult your local building code.
  • Drainage: 4–6 in of gravel under the post keeps water off the end grain.

Questions people actually ask

Straight answers, no fluff. Then run your own numbers above.

How many bags of concrete do I need per post hole?

A 12-inch-diameter hole 36 inches deep holds about 2.36 cubic feet of concrete — that's 4 bags of 80 lb mix, 5 of 60 lb, 6 of 50 lb, or 8 of 40 lb. Deduct the post itself (a 4×4 takes up about 0.26 ft³) and it drops to roughly 2.1 ft³, still 4 × 80 lb bags.

How wide should a post hole be?

Roughly three times the post width: 10–12 inches for a 4×4 post, 14–16 inches for a 6×6. Wider holes in soft or sandy soil give the concrete more bearing surface; in firm clay you can stay near the minimum.

How deep should a post hole be?

At least one-third of the post's above-ground height, and deeper than the frost line in cold climates (12–48 inches depending on region). A 6-foot fence wants 36-inch holes where frost isn't a factor — always check local code before digging.

Should I subtract the post volume from the concrete?

Yes for accuracy. A 4×4 post (3.5 in × 3.5 in) in a 36-inch hole displaces about 0.26 cubic feet — roughly half a bag. The calculator above deducts it automatically.

Bagged concrete or ready-mix for fence posts?

Bags win for a few holes — a fence line of 10–15 holes is usually still under half a yard. Past about 2 cubic yards total (roughly 25+ standard holes), get a ready-mix price; bagging that much by hand is slow and expensive.

Working out a full slab instead? The concrete bag calculator compares bag sizes two ways, and the concrete footing calculator handles strip footings.