Post Hole Concrete Calculator
Setting posts is the foundation of every fence and deck, and the concrete math is a cylinder volume problem: hole diameter × depth × number of posts. Guess small and posts heave in the first winter; guess big and you're hauling 80 lb bags for nothing.
Enter your hole diameter, depth, and post count. Standard rule: hole diameter = 3× the post width, depth = ⅓–½ the above-ground height plus 6 inches of gravel below.
Rule of thumb: 3× the post width (12 in for a 4×4)
⅓–½ of above-ground height; below frost line
- Concrete per hole
- 2.36 cu ft
- Total volume
- 23.6 cu ft
- 80 lb bags
- 40 bags
- 60 lb bags
- 53 bags
- Cost (80 lb bags, est.)
- $200 – $280
Bag counts assume filling the whole hole — you can subtract the displaced post volume (post sq in × depth) if you want the tighter number, and 6 in of gravel in the hole bottom reduces concrete by roughly half a bag per post.
How This Calculator Works
Volume per hole (ft³) = π × (Diameter ÷ 24)² × Depth- Convert hole diameter to radius in feet: diameter in inches ÷ 24.
- Cylinder volume: π × radius² × depth in feet gives cubic feet per hole.
- Multiply by the number of posts for total volume.
- Divide by 0.60 cu ft per 80 lb bag (or 0.45 for 60 lb) and round up.
- Pour 6 inches of gravel in each hole first for drainage, then set the post and fill.
Example: Ten 12 in holes, 3 ft deep: each holds π × 0.5² × 3 = 2.36 cu ft → 23.6 cu ft total → 40 eighty-pound bags.
Frequently Asked Questions
How many bags of concrete per fence post?
A 4×4 post in a 12-inch hole 3 ft deep takes about 2.4 cu ft — four 80 lb bags if you fill the entire hole, though 2 bags plus gravel/tamped backfill also works in firm soil. The common quick answer of "2–3 bags" assumes a 10–12 in hole at 24–30 in depth.
How wide should a post hole be?
Three times the post width: a 4×4 post (3.5 in actual) wants a 10–12 inch hole, a 6×6 wants 12–18 inches. Wider holes at the same depth beat deeper narrow holes for lateral strength — a fence post is a lever, and the concrete collar is the fulcrum.
How deep do post holes need to be?
Bury ⅓ to ½ of the above-ground height, and at least to your local frost line — typically 24–36 inches in most of the US, up to 48+ inches in northern climates. A 6 ft privacy fence wants 30–36 inch holes; decks follow the engineer's spec, usually 12–48 inches.
How much does post concrete cost in 2026?
An 80 lb bag of concrete mix costs about $5.50–$8 at home improvement stores. Setting 10 fence posts at 2–4 bags each runs roughly $110–$320 in concrete — plus a $60–$120 auger rental if you're not digging by hand.
Do I need gravel at the bottom of a post hole?
Yes — 6 inches of gravel under the post drains water away from the buried end, dramatically slowing rot on wood posts. It also gives a firm leveling base. In heavy clay soils, some builders tamp the hole with gravel entirely instead of concrete.
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Last updated: 2026-10-26