Published August 14, 2026 · Updated August 23, 2026 · By Tupep
Start with the hole, not the post
Concrete for a fence post is a donut, not a cylinder. You are filling the space between a round hole and a square post, minus whatever gravel sits at the bottom. Get those three dimensions right and the volume falls out in one line of math.
The three-times-post-width rule for hole diameter is the one figure we checked hardest before building this around it, because it is repeated everywhere without explanation. It exists because the concrete collar has to spread lateral load into undisturbed soil, and a collar narrower than that concentrates the load at the post face. We use it as the default and let you override the diameter, since rocky and sandy ground behave differently.
For the most common case in American residential fencing, a 4×4 wood post in a 10 inch hole 30 inches deep with a gravel base, the answer is almost exactly 1 cubic foot per post, which is two 80 lb bags or three 60 lb bags. That single number gets you most of the way through most fence jobs.
Sizing the hole before you calculate anything
The rule of thumb every fence crew uses is that hole diameter should be about three times the post width. A 4×4 post actually measures 3.5 inches, so three times that is 10.5 inches, and you round to whatever auger you have, usually a 10 or 12 inch. A 6×6 measures 5.5 inches, so you want roughly a 16 inch hole and you round to 15 or 18.
Depth is the part that varies by where you live. A common starting point is burying about one third of the post’s above-grade height, so a 6 foot fence on 9 foot posts wants roughly 3 feet in the ground. Layered on top of that, the hole commonly needs to reach below the local frost depth so the post is not lifted every winter. Frost depth ranges from essentially nothing in the deep South to several feet in northern states, and it is set locally. Call your building department and ask for the frost depth and whether fence posts are regulated in your jurisdiction. Do not take a number off a national chart, and do not take one from us.
The formula
Concrete per post (cu ft) = (3.1416 x r² x d) – (post area x d)
Where r is the hole radius in feet, d is the depth of the concrete in feet, which is the hole depth minus the gravel base, and post area is the actual cross section in square feet. For a 3.5 inch post that is 3.5 x 3.5 = 12.25 square inches, divided by 144, which is 0.0851 square feet.
Worked example
A 4×4 post, 10 inch hole, 30 inches deep, with 4 inches of gravel at the bottom for drainage. Concrete depth is 26 inches, which is 2.167 feet.
- Hole radius: 5 inches = 0.4167 ft. Squared is 0.1736. Times pi is 0.5454 square feet of hole area.
- Gross concrete volume: 0.5454 x 2.167 = 1.182 cubic feet.
- Post displacement: 0.0851 x 2.167 = 0.184 cubic feet.
- Net concrete: 1.182 – 0.184 = 0.998 cubic feet per post.
At 0.60 cubic feet per 80 lb bag that is 1.66 bags, so you buy two per hole. At 0.45 per 60 lb bag it is 2.2, so three per hole if you mix hole by hole.
| Post | Hole dia | Hole depth | Concrete depth | Cu ft per post | 80 lb bags | 60 lb bags |
|---|---|---|---|---|---|---|
| 4×4 (3.5 in) | 10 in | 30 in | 26 in | 1.00 | 2 | 3 |
| 4×4 (3.5 in) | 12 in | 36 in | 32 in | 1.87 | 4 | 5 |
| 4 in round steel | 10 in | 30 in | 26 in | 0.99 | 2 | 3 |
| 6×6 (5.5 in) | 15 in | 36 in | 32 in | 2.71 | 5 | 7 |
| 6×6 (5.5 in) | 18 in | 42 in | 38 in | 4.93 | 9 | 11 |
Notice that a 4 inch round steel post and a 4×4 wood post displace nearly the same volume, so you can use one number for a mixed fence.
Scaling to a whole fence
A 100 foot run with posts at 8 feet on center needs 13 bays, so 14 posts including both ends. At 1.0 cubic foot each that is 14 cubic feet, which is 24 bags of 80 lb, or 26 with a waste allowance.
Buying two bags per hole would put you at 28. The gap is real money on a long fence, and the way we close it is by mixing in a tub or a mixer for three or four holes at a time instead of one bag at a time. Mixing per hole always rounds up, and those rounded-up half bags add up across a property line.
Corner posts, end posts, and gate posts take more. They carry lateral load that line posts do not, so they commonly get a hole one size larger and several inches deeper. Budget an extra bag apiece and you will not be short.
Details that matter in the hole
- Gravel at the bottom. Three or four inches of compacted crushed stone lets water drain away from the post end. Fully encasing a wood post in a concrete cup traps water and rots it from the bottom up.
- Crown the top. Finish the concrete an inch or two above grade and slope it away from the post in every direction. A flat or dished top holds a puddle right against the wood.
- Fast setting post mix. The bagged fast setting products are designed to be poured in dry and then wetted, typically around a gallon of water per 50 lb bag, and they usually take a set in well under an hour. Read the bag, because the water volume and the wait before framing are product specific.
- Post mix yields differ. Fast setting mixes do not always yield the same per pound as standard mix. Check the stated yield on the bag rather than assuming 0.60.
One boundary worth naming. This is fence math. If the post is carrying a roof, a deck, a carport, or a pergola with any real span, that is a structural footing, not a fence hole, and it needs a designed size and usually a permit and an engineer.
Related guides
- Concrete for a 4×8 Patio
- Gravel Base Depth Under a Paver Patio
- Bags vs Ready-Mix: Where the Break-Even Actually Is
- French Drain Gravel Quantity
Sources and Further Reading
Run your own numbers with the Concrete Slab Calculator, or browse every tool on the calculators index. Coverage figures and material constants are cross-checked against manufacturer technical data sheets before publication. See our Methodology page for how figures are selected and verified.
Planning estimate only. Results assume standard conditions and are intended for material planning. Verify against your product’s technical data sheet and your local building code before purchasing. Structural, electrical, and gas work should be specified by a licensed professional.
Last updated: August 23, 2026
Leave a Reply