Fence Post Concrete Calculator
Enter the hole diameter, the depth and the number of posts, and this fence post concrete calculator returns the bags you need in 40, 60 and 80 pound sizes, plus the total volume. It works for fence posts, deck posts, mailbox posts and sign posts set in round holes.
Fence Post Concrete Calculator
A post hole is a cylinder, so the volume is pi times the radius squared times the depth. The number most people want is bags per post, which is the hole volume divided by the yield of one bag. The tool repeats that across every post in your run.
How to calculate concrete for a fence post
First find the volume of one hole. Diameter is across the top, so the radius is half of that. With the diameter in inches and the depth in inches, convert to cubic feet at the end. Then subtract the volume the post itself takes up, because the post sits in the hole and displaces concrete.
A common setup is a 10 inch wide hole at 24 to 36 inches deep for a 4 by 4 post. One 10 inch hole at 30 inches deep holds about 1.36 cubic feet before subtracting the post, which is close to three 80 pound bags.
How deep should a post hole be
Two rules set the depth. A fence post should sit about one third of its above ground height in the ground, so a 6 foot fence often uses a hole 2 to 3 feet deep. The hole bottom should also reach below the frost line in cold regions, the same depth rule used for footings. Deeper, wider holes hold a post against wind and frost heave.
| Post | Hole diameter | Hole depth | 80 lb bags per post |
|---|---|---|---|
| Mailbox or light post | 8 in | 24 in | 2 |
| 4 × 4 fence post | 10 in | 24 in | 2 |
| 4 × 4 fence post | 10 in | 36 in | 3 |
| 6 × 6 deck post | 12 in | 36 in | 4 to 5 |
Worked example, a fence line
A 100 foot fence with posts every 8 feet needs about 13 posts. Using a 10 inch hole 30 inches deep, each hole holds about 1.36 cubic feet, so 13 posts come to roughly 17.7 cubic feet. At 0.60 cubic feet per 80 pound bag that is about 30 bags, or 3 bags per post rounded up. Buy a few extra, since holes dug by hand or auger tend to bell out at the bottom and hold more than the nominal size.
Tips for setting posts
- Add a few inches of gravel at the bottom of each hole for drainage so water does not pool around the post.
- Keep the post plumb and braced while you pour and until the concrete sets.
- Crown the top of the concrete so water sheds away from the post.
- Fast setting mixes let you set posts without mixing water separately, which speeds a long fence line.
Related calculators
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- Footing calculator for pad footings and frost depth
- Bags calculator to total bags across a project
- How to calculate concrete guide for formulas
Frequently asked questions
How much concrete do I need for a fence post?
A 10 inch hole 30 inches deep holds about 1.36 cubic feet, roughly three 80 pound bags. Multiply by the number of posts and round up.
How many bags of concrete per fence post?
Most 4 by 4 posts in a 10 inch hole take 2 to 3 bags of 80 pound mix, depending on depth. A 6 by 6 deck post in a 12 inch hole takes 4 to 5.
How deep should a fence post hole be?
About one third of the post height above ground, and below the frost line in cold regions. A 6 foot fence often uses a 2 to 3 foot hole.
How wide should a post hole be?
Roughly three times the post width. A 4 by 4 post uses a hole around 10 inches across; a 6 by 6 post uses about 12 inches.
Do I need gravel under a fence post?
A few inches of gravel at the bottom helps water drain away from the post base, which slows rot and frost damage.
Can I use fast setting concrete for posts?
Yes. Fast setting mix is common for fence posts because you can pour it dry into the hole and add water, with no separate mixing.
References
- Quikrete, Concrete Mix 1101 data sheet (bag yields): https://www.quikrete.com/PDFs/DATA_SHEET-Concrete Mix 1101.pdf
- International Residential Code R403 footings and frost depth, via InterNACHI: https://www.nachi.org/foundation-footings.htm
- Portland Cement Association, concrete technology library: https://cement.org
Methodology: hole volume uses pi times radius squared times depth, with post displacement subtracted, then divided by published bag yields and rounded up. Hand dug and augered holes often hold more than the nominal size, so carry extra. Confirm depth against local frost and code requirements. Recommended: attribute this page to a named author and add a review date.