Fence Post Concrete Calculator
Wondering how many bags of concrete per fence post you need? Our free fence post concrete calculator figures it out for you: enter your hole diameter and depth, pick your post size, and set the number of posts. You'll get the concrete volume per post, the bag count for 60-lb and 80-lb bags, and the total bags for your whole fence — with a waste allowance built in so you don't run short.
Enter your numbers
Your results
| Bag size | Per post | Fence total |
|---|---|---|
| 80-lb bags | — | — |
| 60-lb bags | — | — |
How to use this calculator
Measure one of your post holes — the diameter across the top and the depth — and enter them in inches. Choose your post size: a standard 4×4 (which is actually 3.5 inches square) or a 6×6 (actually 5.5 inches square), or pick "round post" and enter the post's diameter. Then enter your number of posts and leave the 10% waste allowance on. Results update instantly as you type.
The per-post bag counts are rounded up to whole bags, and the fence total is the per-post count times your number of posts — so you'll buy enough to finish every hole. If you're setting posts in varying hole sizes, run the calculator for each size separately and add the totals.
How it works
Each hole is treated as a cylinder: hole volume = π × (diameter ÷ 2)² × depth, in cubic inches. The buried post is treated as a solid prism or cylinder and subtracted, because it displaces concrete: net volume = hole volume − post cross-section × depth. Dividing by 1,728 converts cubic inches to cubic feet, and the waste multiplier (1 + waste ÷ 100) is applied on top.
Bag counts use standard bag yields — an 80-lb bag yields about 0.60 cu ft and a 60-lb bag about 0.45 cu ft — with bags = ceil(volume with waste ÷ yield per bag), always rounded up. Results are planning estimates; actual yield varies slightly by mix brand and how wet you mix it.
Frequently asked questions
How many bags of concrete do I need per fence post?
It depends on the hole size. For the most common setup — a 10-inch-diameter hole, 24 inches deep, with a 4×4 post — you need about 2 eighty-pound bags or 3 sixty-pound bags per post, including a 10% waste allowance. Enter your exact hole and post dimensions above for your number.
Should I subtract the post volume?
Yes. The buried part of the post displaces concrete, so a calculator that ignores it overestimates what you need. A buried 4×4 post displaces roughly 0.17 cubic feet in a 24-inch-deep hole. This calculator subtracts the post's cross-section from the hole volume automatically.
60 lb or 80 lb bags — which should I buy?
Both contain the same concrete mix; only the bag weight differs. An 80-lb bag yields about 0.60 cubic feet and a 60-lb bag about 0.45 cubic feet. For a whole fence, 80-lb bags mean fewer bags to buy, carry, and mix — usually the better deal. Choose 60-lb bags if you prefer lighter lifting.
How deep should a fence post hole be?
A common rule is one-third of the post's above-ground height, with a minimum of 24 inches for most fences, plus a few inches of gravel at the bottom. In cold climates, dig below the frost line so freeze-thaw cycles don't heave the posts. Check local building codes if your fence requires a permit.