Calculator

Concrete Post Calculator

Post-hole concrete uses a different formula shape from a slab, and it's worth stating explicitly so the two calculators aren't confused: a slab is one flat volume, but a post hole's concrete volume is the hole's cylindrical volume minus the volume the post itself displaces, because the post sits inside the hole and concrete only fills the space around it.

This calculator works from hole diameter and depth, subtracts the post's own footprint, and returns bags per hole — cross-reference the fence calculator for how many post holes your total fence run needs.

Calculate your quantity

Hole diameter (in)

Hole depth (ft)

Post width (in)

Post depth (in)

Number of posts

Total concrete

0.87 cu ft

Per hole: 0.87 cu ft
60lb bags: 2 bags
80lb bags: 2 bags
90lb bags: 2 bags

Shopping summary

• 2 x 80lb bags for 1 post hole(s)

This is an estimate — confirm structural work with a professional.

How this calculation works

Hole volume (cubic feet) = pi x (radius in feet)^2 x depth in feet, using the hole's diameter, not the post's. Post displacement = pi x (post radius)^2 x the depth the post is buried. Net concrete volume = hole volume minus post displacement volume, then converted to bags the same way as the slab calculator (0.45 cu ft/60lb bag, 0.6 cu ft/80lb bag).

A common rule of thumb for hole depth is roughly a third of the post's above-ground height, plus a few inches of gravel at the bottom for drainage — burying a post too shallow risks frost heave in cold climates and wind-load failure on tall fence or deck posts.

The formula

perHole = max(0, π×(holeDiameter/24)² × depth − (postWidth/12)×(postDepth/12)×depth); total = perHole × postCount

holeDiameter/24: Hole RADIUS in feet from a diameter entered in inches — divided by 24, not 12, because the value is halved for radius and converted from inches to feet in the same step.
(postWidth/12)×(postDepth/12)×depth: The post's own volume, treated as a rectangular prism the full depth of the hole — this is subtracted whole, so a post that only extends partway down a deeper hole is not modeled; depth here is shared between the hole and the post assumption.
max(0, ...): A floor at zero for the per-hole volume — if a post's cross-section were ever entered larger than the hole's, the function reports no concrete needed rather than a negative number, which is a sign the post and hole dimensions don't fit together, not a valid result to act on.
postCount: A flat multiplier applied to one uniform hole size — every hole in the run is assumed identical, so a fence with deeper corner-post holes than line-post holes needs this calculator run twice and the two totals added by hand.

Where these numbers come from

hole volume as a cylinder, post volume as a rectangular prism: Derived on this page from the geometry the function actually uses: a round auger hole is genuinely cylindrical, and a dimensional post is genuinely square or rectangular in cross-section, so the two shapes are modeled differently rather than both approximated as one or the other.
a third of post height as hole depth, plus drainage gravel: Trade convention for setting a fence or deck post; this calculator does not enforce or check that ratio, so entering a depth is the user's structural judgment call, not a validated input.
60 / 80 / 90 lb bag yields shared with the slab calculator: Manufacturer-published bag yields, the same figures the concrete slab calculator uses — post-hole and slab pours draw from the identical bag-yield table, only the volume formula ahead of it differs.

Worked examples

A single 8 inch hole for a mailbox or light post

Inputs
Hole diameter	8 in
Hole depth	1.5 ft
Post width	4 in
Post depth	4 in
Number of posts	1

Result
Total concrete	0.36 cu ft
Per hole	0.36 cu ft
60lb bags	1 bags
80lb bags	1 bags
90lb bags	1 bags

An 8 inch, 1.5 ft deep hole holds only 0.52 cu ft before the post is subtracted, and a nominal 4×4 post (real dimensions closer to 3.5×3.5 inches, entered here at its full 4 inches) removes 0.17 cu ft of that, leaving 0.36 cu ft — one bag of any size covers it with room left over.

This is the case where the post-versus-slab formula difference matters least in practice: the post takes up nearly a third of a hole this small, but the whole job is still a single bag either way, so the subtraction step is more about accuracy than about avoiding a real shortfall.

Four 14 inch holes for a pergola, deeper and wider

Inputs
Hole diameter	14 in
Hole depth	3.5 ft
Post width	6 in
Post depth	6 in
Number of posts	4

Result
Total concrete	11.47 cu ft
Per hole	2.87 cu ft
60lb bags	26 bags
80lb bags	20 bags
90lb bags	17 bags

Each hole here is 2.87 cu ft net after subtracting a 6×6 post, and four of them total 11.47 cu ft — 20 bags of 80 lb mix. Compare that per-hole figure to the mailbox post above: nearly 8 times the net concrete for less than double the diameter, because hole volume scales with the RADIUS squared, not the diameter.

That squared relationship is the real reason a pergola or deck footing order looks disproportionate next to a single fence post: going from an 8 inch to a 14 inch hole almost triples the radius-squared term even before the extra depth and the fourth hole are factored in.

Common mistakes

Reusing the slab calculator's flat-volume formula for post holes, which overstates bag count since it doesn't subtract the post's own volume.
Digging holes too shallow for the post height and local frost line, which is a structural problem, not just a concrete-quantity one.
Skipping the gravel drainage layer at the hole bottom, which can trap water against the post base and accelerate rot or corrosion.
Not accounting for multiple holes at different depths (e.g., corner posts often go deeper than line posts) when totaling bag count for a whole fence run.
Setting the post out of plumb before the concrete sets — once concrete begins to harden, correcting a leaning post means chipping it back out rather than a quick adjustment.
Backfilling with dry mix and relying on ground moisture to cure it ('dry-set' method) in a climate or soil condition it isn't well suited for — this technique works in some conditions but isn't a universal substitute for properly mixed concrete.

Shopping summary

Buy whole bags per hole from the net-volume formula above, multiplied by total hole count, plus a bag of drainage gravel per hole for the base layer — this is a structural install; confirm depth and post spacing against your local frost line and any HOA/permit requirements before digging.

Worked example: a hole 10 inches in diameter and 24 inches deep has a volume of roughly pi x (5/12)^2 x 2 ≈ 1.09 cubic feet. A 4x4 post (actual size roughly 3.5x3.5 inches) buried 24 inches deep displaces about 0.17 cubic feet. Net concrete volume is about 0.92 cubic feet — just under two 60lb bags (0.45 cu ft each) or roughly 1.5 80lb bags, so round up to two bags of whichever size you're using per hole.

FAQ

Why isn't post-hole concrete just hole volume?

Because the post occupies real volume inside the hole — a 4x4 or 6x6 post displaces a noticeable fraction of a typical hole's volume, and skipping that subtraction means over-ordering bags for every hole in the project.

How deep should a fence post hole be?

A common guideline is about a third of the post's total length (so a 6-foot fence post gets roughly a 2-foot hole), adjusted deeper in regions with a significant frost line, plus a few inches of gravel below the post base for drainage.

Should I crown the concrete above grade around the post?

Yes, typically — sloping the top of the concrete away from the post so water runs off rather than pooling against the post base is standard practice and helps prevent moisture from wicking up into the post over time, independent of how much concrete volume was used below grade.

Do larger posts (6x6) need a proportionally bigger hole than 4x4 posts?

Generally yes — a heavier, larger post carries more load and benefits from a wider hole diameter for adequate concrete surrounding it, not just a hole sized to the post's own footprint plus a thin margin; check span and load tables for your specific application rather than scaling the 4x4 hole size up by eye.

Can I use quick-setting post-mix concrete instead of standard mix?

Yes — many fence and deck post installs use a quick-setting product specifically formulated to be poured dry into the hole and set with the water already in the surrounding damp soil, which skips a separate mixing step, though it's worth checking that product's specific yield-per-bag figure rather than assuming it matches standard general-purpose mix.

Where to go next

The projects this number is a step of, the guides that explain the method behind it, and the rest of its trade group.