Fence Post Concrete Calculator
Enter your post count, hole size, and depth to get the exact number of concrete bags — no more guessing a bag per post and running short.
Rule of thumb: 3× the post width.
⅓ of the above-ground fence height, below the frost line.
Hole volume minus the post itself.
How it works
We compute each hole's cylinder volume, subtract the post that displaces concrete, and multiply by your post count. Bag counts use standard yields: an 80-lb bag makes about 0.6 cubic feet, 60-lb about 0.45, and 50-lb about 0.375.
Formula: Per hole: π × (dia÷2)² × depth − post volume; Bags = total ft³ ÷ yield (80 lb ≈ 0.6 ft³)
Good for
- Wood and vinyl fence runs
- Mailbox and lamp posts
- Deck stair rail posts
- Gate posts (upsize the hole)
Tips
- Set posts at least ⅓ of the fence height deep — 24″ minimum for a 6-ft fence, 30″ in windy sites.
- Bell the bottom of the hole slightly wider than the top to resist frost heave.
- Add 6″ of gravel below the post for drainage before pouring.
- Fast-setting mix can be poured dry into the hole and watered — check the bag directions.
References
Estimates use standard industry volume math. Cross-check your numbers against the pros:
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Open calculator →Fence Post Concrete Calculator FAQ
How many bags of concrete per fence post?
A typical 4×4 post in a 12″-wide, 24″-deep hole takes about 1.4 cubic feet of concrete — roughly 2–3 of the 80-lb bags per post, not the single bag many people assume.
How deep should fence post holes be?
At least one-third of the fence height, with 24 inches as a practical minimum for a 6-ft privacy fence — and always below your local frost line so posts don't heave.
How wide should the hole be?
About three times the post width: a 4×4 post (3.5″ actual) wants a 10–12 inch hole so there's a proper collar of concrete on every side.