Fence Post Concrete Calculator
Concrete per post hole and total bags for a fence, with embedment depth checked against the one-third rule and a frost depth warning.
How much concrete per hole, how many bags for the whole fence, and how deep the hole should be — the depth rule is the part people skip and then rebuild.
Quick answer: concrete bags for 12 fence posts
| Hole | Concrete per hole | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 8 in. × 24 in. deep | 0.44 ft³ | 10 | 13 |
| 10 in. × 30 in. deep | 0.997 ft³ | 22 | 30 |
| 10 in. × 36 in. deep | 1.228 ft³ | 28 | 37 |
| 12 in. × 36 in. deep | 1.868 ft³ | 42 | 55 |
| 12 in. × 42 in. deep | 2.218 ft³ | 49 | 66 |
4 × 4 post (3½ in.), 4 in. of gravel in the bottom, 10 % waste; an 80 lb bag yields 0.60 ft³, a 60 lb bag 0.45 ft³. The frost line where you live sets the depth, not this table.
| Concrete per hole | — |
|---|---|
| Total concrete | — |
| Bags needed | — |
| Hole depth vs post height | — |
| Post embedded | — |
Below the frost line is necessary, not sufficient. NRC Canada's CBD‑128 opens with exactly that, for frost‑susceptible soil. Frozen ground bonds to the sides of whatever passes through it and lifts. Penner and Burn measured that grip, called adfreezing, at 12.5 psi on steel posts in clay, and the digest carries 18 psi for design. Uplift is frost depth × perimeter × that stress. A 10‑inch collar through 42 inches of frost is 42 × 31.4 × 18, about 23,700 pounds. That is twelve tons pulling up against two and a half bags of concrete, under 200 pounds, holding down. One more bag is not the fix.
Uplift scales with perimeter, so the wider hole is the worse one. Going from 10 inches to 12 adds 20% to the grip and 52% to the concrete. The three‑times‑post‑width flag above and the one‑third‑depth rule beside it are lateral rules. IBC 1807.3.2.1 sizes post embedment against wind pushing the post over, not frost pulling it out. CBD‑128's fixes are geometric instead. One is an anchor footing sized for the uplift, and its 3½‑inch test piles moved 0.02 feet under 6,000 pounds on 2‑foot‑square pads. The other is a drained granular zone up the sides. That zone must actually drain; saturated, the digest warns, it will freeze into a solid mass and pass the heave through anyway. The four inches under your post is drainage, not that zone.
Nobody is going to make you dig that deep. Minnesota Rule 1303.1600 sets footings at 5 feet in its northern counties and 3½ feet in the rest; this page opens at 30 inches, short of both. IRC R105.2 exempts fences not over 7 feet from permit, so no inspector will ever look at yours.
What I check on a job
Measure the hole, then buy the bags. The diameter I type in is the tool's, not the hole's. A hand-dug hole is rarely a clean cylinder, and every extra inch of diameter is concrete the page never counted. So after the first two holes I measure what was actually dug and rerun the numbers before buying for the rest. The gravel field cuts the other way. The page subtracts whatever base I enter, so if less gravel goes into the bottom of the hole than I typed, the difference fills with concrete I did not order. I also check the bag on the pallet against its own data sheet, because the 0.60 and 0.45 cubic-foot yields here are Quikrete's published figures and a different mix need not match them. Depth I enter last, once I have the local figure. Only then do I add the page's 10 percent and place the order.
Concrete per hole is the hole cylinder minus the post and minus the gravel base:
π(d÷2)² × (depth − gravel) − post area × (depth − gravel).
Bag yields are Quikrete's published figures (Technical Data Sheet #1101). An 80 lb bag yields
0.60 ft³, a 60 lb bag yields 0.45 ft³.
A common rule of thumb puts roughly a third of the total post length in the ground, and hole diameter
at about three times the post width. Frost depth overrides both. In a freezing climate the footing must
extend below the local frost line or the post will heave, and that depth is set by your building
department, not by a rule of thumb. Check it before you dig.
Worked example
Twelve 4×4 posts in 10-inch holes, 30 in deep with 4 in of gravel: how many bags?
- Concrete depth = 30 − 4 = 26 in.
- Per hole: π × 5² × 26 − 3.5² × 26 = 1,723 in³ = 0.997 ft³.
- 12 holes = 11.97 ft³; +10% = 13.2 ft³; 80 lb bags yield 0.60 ft³: 22 bags.
22 bags of 80 lb, about one ft³ a hole. A 30-inch hole is short of any northern frost depth — the page says so — and that overrides the arithmetic.
8 values on this page were driven in a browser and compared with independently derived figures on 2026-10-29 (2808 values across 228 calculators site-wide).
Questions
How many bags of concrete per fence post?
About two 80 lb bags for a 10-inch hole 30 inches deep with 4 inches of gravel: roughly 1 ft³ of concrete a hole, 22 bags for twelve posts with a 10% allowance.
How deep should a fence post go?
A third of the total post length is the rule of thumb, but frost depth overrides it: the footing must reach below the local frost line, and that figure comes from your building department.
Why does more concrete not stop frost heave?
Because frozen ground grips the sides of the collar and lifts it (adfreezing; NRC Canada's CBD-128 carries 18 psi for design). A 10 in. collar through 42 in. of frost takes about 42 × 31.4 × 18 = 23,700 lb of uplift, against under 200 lb of concrete holding it down.
Does a wider hole help against frost?
No, it makes it worse: uplift scales with perimeter, and going from 10 in. to 12 in. adds 20% to the grip and 52% to the concrete. CBD-128's fixes are geometric: an anchor footing sized for the uplift, or a granular zone up the sides that really drains.