Home › Fence Concrete Calculator
Fence Concrete Calculator
Quick answer: Concrete per post equals the hole volume (π × radius² × depth) minus the post's own volume. Multiply by the number of posts, then divide by the yield printed on your bag — often about 0.6 cubic feet for an 80 lb bag of premix. Check your bag label, because yields differ by product and bag size.
How it works
Hole volume in cubic inches = π × (diameter ÷ 2)² × depth; subtract post width² × depth for a square post; divide by 1,728 for cubic feet. Total = per-hole volume × posts. Bags = total ÷ bag yield, rounded up. Some installers use gravel at the base or set posts in compacted gravel instead; follow your installer or manufacturer guidance.
Worked example
Figures below are illustrative examples (2026-10-29), not quotes or guarantees.
Assumptions: 20 posts, 10 in holes, 36 in deep, 4x4 post (3.5 in actual), 0.6 cu ft per bag. Each hole needs about 1.39 cu ft; total about 27.6 cu ft, or 47 bags. Wider 12 in holes would push that above 70 bags.
FAQ
How deep?
See how deep fence posts and confirm with your municipality.
Fast-setting or regular?
Follow the bag directions; the math here is the same.
How many posts?
Use the fence post calculator.
Back to the main calculator · Guides & FAQ