Fence Post Concrete Calculator
Enter your post count and hole size to get the exact number of 60 lb and 80 lb concrete bags — no half-empty bags, no second trip.
↺ Post count filled in from the fence in your last calculation. Edit it if needed.
How much concrete does a fence post need?
The concrete you need is simply the volume of the hole minus the volume of the post sitting in it. This calculator computes the hole as a cylinder (π × radius² × depth), subtracts the buried part of the post, then converts to bags using real yields: a 60 lb bag makes about 0.45 cubic feet and an 80 lb bag about 0.6 cubic feet of set concrete.
Because it always rounds up to whole bags, the number you see is what to buy. Concrete is cheap and running short mid-pour is not, so a spare bag is never wasted money.
The two numbers that matter most
- Hole depth is set by structure and frost. Go at least one-third of the fence height into the ground and always below the local frost line — a shallow post heaves and leans within a season.
- Hole diameter drives concrete use the hardest, because volume grows with the square of the radius. Doubling from 8 to 16 inches uses roughly four times the concrete.
Corner, end and gate posts
These carry more load. Dig them deeper (add 6 inches) and often wider, then run that larger hole size through the calculator separately so you do not under-buy. A sagging gate almost always traces back to an under-set post.
Estimates only — follow the concrete manufacturer's instructions and your local code for post depth, footing size and permits.
Frequently asked questions
How much concrete do I need per fence post?
A typical 4×4 post in an 8-inch-wide, 24-inch-deep hole needs about 0.6 cubic feet of concrete — roughly one and a half 60 lb bags or one 80 lb bag per post. Wider or deeper holes need proportionally more.
How deep should a fence post be set?
Set posts at least one-third of the above-ground height into the ground, and always below your local frost line. For a 6-foot fence that usually means a 24- to 30-inch-deep hole. Gate and corner posts should go deeper.
How wide should the post hole be?
A common rule is three times the post width — about 10 to 12 inches for a 4×4. An 8-inch hole works for light fences; wider holes hold better in loose or sandy soil but use much more concrete.
Is a 60 lb or 80 lb bag better?
They contain the same mix — an 80 lb bag just yields more (about 0.6 ft³ vs 0.45 ft³ for a 60 lb bag). Fewer 80 lb bags means less handling, but they are heavier to carry. This calculator gives you the count for both.