Post Hole Concrete Calculator

Enter your hole diameter, hole depth, and post size (the width of a square post or the diameter of a round post), then set the number of posts. The calculator finds the concrete volume for each post hole, subtracts the space the post takes up, and tells you how many bags of concrete to buy. By default it assumes a 4x4 square post, 80-lb bags, and imperial (inch) measurements — the most common backyard-fence setup, so you get an answer with just four numbers. Open More options to switch to metric, choose a round post, pick 60-lb or 40-lb bags, add a waste allowance, or enter a price per bag for a quick cost estimate.

The concrete that fills a post hole is simply the hole minus the post. The hole is treated as a cylinder, so its volume is π × (radius)² × depth. The post occupies part of that space, so we subtract the post's volume over the same depth. What's left is the concrete you actually pour.

Most people ask 'how many bags of concrete per post hole?' rather than cubic yards, so this tool converts the volume into whole bags. A standard 80-lb bag of pre-mixed concrete yields about 0.60 cubic feet, a 60-lb bag about 0.45 cubic feet, and a 40-lb bag about 0.30 cubic feet. Bags are always rounded up — you can't buy two-thirds of a bag — and it's smart to add a little extra for spillage and settling.

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Hole volume = π × (hole diameter ÷ 2)² × hole depth
Post volume = post size² × hole depth (square post)
= π × (post diameter ÷ 2)² × depth (round post)
Concrete per hole = Hole volume − Post volume
Total concrete = Concrete per hole × posts × (1 + waste%)
Bags = round up( Total cubic feet ÷ bag yield )
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Bag yield: 80-lb ≈ 0.60 ft³, 60-lb ≈ 0.45 ft³, 40-lb ≈ 0.30 ft³. One cubic yard = 27 cubic feet, so a full yard takes roughly 45 bags of 80-lb concrete.

Example 1 — a 6-foot wood fence. You set five 4x4 posts in 12-inch-wide holes, 24 inches deep. Each hole is π × 6² × 24 = 2,714 in³; the post takes 4 × 4 × 24 = 384 in³, leaving 2,330 in³ per hole. Across five holes that's about 6.74 cubic feet, or 12 bags of 80-lb concrete.

Example 2 — a single mailbox post. One 4x4 post in a 10-inch hole, 24 inches deep, needs about 0.9 cubic feet of concrete — 2 bags of 80-lb mix with a little to spare.

Example 3 — a heavy gate post. A 6x6 post in a 16-inch hole, 36 inches deep, uses roughly 3.4 cubic feet — about 6 bags of 80-lb concrete for that one hole.

- Hole width: make the hole about three times the post's width — a 4x4 post wants a 10–12 inch hole. Wider holes use more concrete but resist leaning better.
- Depth: bury at least one-third of the post. A 6-foot fence needs holes about 2 feet deep; go deeper in areas with frost or high wind.
- Add a gravel base: 4–6 inches of gravel at the bottom improves drainage and slows rot — remember it slightly reduces the concrete depth.
- Buy 10% extra: holes are rarely perfect cylinders, so a waste allowance keeps you from a second trip to the store.
- Fast-setting mix: for fence posts you can often pour dry, fast-setting concrete straight into the hole and add water — always check the bag's instructions first.