Post Hole Concrete Calculator
Enter your hole diameter and depth, the post size, and how many holes — the tool subtracts the post from each hole's cylinder and totals the concrete in cubic feet and whole bags. Hole dimensions are yours; QUIKRETE's 3×-post-width suggestion appears below as labeled manufacturer guidance, never a code requirement.
Hole, post, and count
Example: eight 12 in × 24 in holes with 4×4 posts (3.5 in dressed) at 10% waste → 11.21 cu ft, 33 bags of 50 lb mix.
Guidance, not code: QUIKRETE's setting-posts guide suggests a hole 3× the post width, 1⁄3–1⁄2 of the above-ground post height deep, over 6 in of gravel — a manufacturer's suggestion to start from. Frost depth, embedment, and anything structural are set by your local building department or engineer.
Hole minus post, times holes
The worked example: a 12 in hole is a 1 ft cylinder, so 24 in of depth holds π ⁄ 2 = 1.57 cu ft. A dressed 4×4 (3.5 in per the PS 20 lumber standard) displaces 0.17 cu ft, leaving 1.4 cu ft per hole. Eight holes need 11.21 cu ft; with 10% extra that is 12.33 cu ft — 33 bags of 50 lb mix at the manufacturer's 0.375 cu ft yield. Computed at build time by the same engine wired to the form.
What is sourced, what is yours
Three cited facts and nothing else: the bag yields (QUIKRETE No. 1101 data sheet and the Canadian metric figures), the dressed lumber sizes (PS 20 American Softwood Lumber Standard — why a 4×4 defaults to 3.5 in), and QUIKRETE's hole-sizing guidance, which is labeled as a manufacturer suggestion and never auto-applied. Hole diameter, depth, count, waste, and price are all yours.
Fences, footings, and the rest
Setting a whole fence line? The fence calculator counts posts, rails, and pickets and reuses this exact hole math for the setting concrete. Strip footings are the concrete footing calculator's job, formed piers the concrete column calculator's, and the concrete bag calculator compares every bag size for any volume.
Frequently asked questions
How big should the hole be?
That is your call — the tool computes volume for whatever you enter. As labeled guidance, QUIKRETE's setting-posts guide suggests a hole 3× the post width (about 12 inches for a 4×4) and a depth of one third to one half of the post's above-ground height, plus 6 inches for a gravel base. That is the manufacturer's installation guidance, not a building code: frost depth and embedment requirements come from your local code official, and fence or structure loads may demand more.
Why is the post subtracted from the hole volume?
Because the post occupies space concrete cannot fill. A 12 in × 24 in hole holds π/2 ≈ 1.57 cu ft, but a 4×4 post (actually 3.5 × 3.5 in — the dressed size per the American Softwood Lumber Standard) displaces 0.17 cu ft of that, leaving about 1.40 cu ft of concrete per hole. Skipping the subtraction over-buys roughly one bag in ten on typical fence holes.
Why is a 4×4 post really 3.5 inches?
Lumber is sold by nominal size but surfaced (planed) to a standard dressed size: PS 20, the American Softwood Lumber Standard, sets the dry dressed size of nominal 4 in at 3½ in and nominal 6 in at 5½ in. The tool defaults to the dressed 3.5 in for a 4×4 and lets you type any actual size — including true rough-sawn 4 in stock.
Does the gravel base change the concrete amount?
Yes — if you pour 6 inches of gravel first, the concrete only fills the remaining depth, so enter the concrete depth (hole depth minus the gravel), not the full dug depth. The tool computes exactly what you give it.
Can this size a structural post or deck footing?
No. Post embedment, footing diameter, and frost depth for anything structural — decks, pergolas, buildings — are code-governed design questions for your building department or engineer. This tool only totals concrete for hole dimensions you already have.
Dimensions never leave your browser. Sources and the estimating pipeline are documented on the methodology page.