How it works
A post hole is a cylinder, and the post takes up some of it, so the calculator subtracts the post before working out bags.
QUIKRETE's guides say to dig the hole about 3 times the post width and about 1/3 of the post's overall length deep, with 6 in of gravel at the bottom for drainage. Posts that carry a structural load must follow local code, often below the frost line.
For a whole fence, see the fence calculator for posts, panels and gates.
The formula
- Concrete per hole = (π × (hole diameter ÷ 2)² − post width²) × concrete depth ÷ 1,728 (cu ft)
- Total = concrete per hole × posts × (1 + extra %)
- Bags = ⌈total ÷ yield per bag⌉
- Cost = bags × price per bag
How to use this calculator
- Count the posts.
- Enter the hole diameter and how deep the concrete will be.
- Enter the post's actual width.
- Set the yield for the bag you'll use and your price.
Worked example: ten 4×4 posts in 12 in holes
- Hole area: π × 6² = 113.1 sq in. Post: 3.5 × 3.5 = 12.25 sq in. Concrete area: 100.85 sq in.
- At 24 in of concrete: 100.85 × 24 ÷ 1,728 = 1.40 cu ft per hole.
- Ten posts with 10% extra: 1.40 × 10 × 1.10 = 15.4 cu ft.
- 50 lb Fast-Setting bags at 0.375 cu ft: 15.4 ÷ 0.375 = 41.1, so about 42 bags (about 5 per hole if you buy per hole). Rough estimate, not a guarantee.
Bags of concrete per fence post
| Post and hole | Concrete depth | Concrete per hole | 50 lb Fast-Setting bags | 80 lb Concrete Mix bags |
|---|---|---|---|---|
| 4×4 post, 10 in hole | 24 in | 0.92 cu ft | 3 | 2 |
| 4×4 post, 12 in hole | 24 in | 1.40 cu ft | 4 | 3 |
| 4×4 post, 12 in hole | 30 in | 1.75 cu ft | 5 | 3 |
| 6×6 post, 16 in hole | 30 in | 2.97 cu ft | 8 | 5 |
| 6×6 post, 18 in hole | 36 in | 4.67 cu ft | 13 | 8 |
Rough estimate, not a guarantee. Per hole, rounded up, no extra added, using QUIKRETE's listed yields (0.375 cu ft per 50 lb Fast-Setting bag, 0.60 cu ft per 80 lb bag). Real holes are rarely perfect cylinders.
Frequently asked questions
How many bags of concrete per fence post?
For a 4×4 post in a 10 in hole with 24 in of concrete, about 3 bags of 50 lb Fast-Setting mix. QUIKRETE's own data sheet says two 50 lb bags set a 4 in post in a 10 in hole 2 ft deep, so it is worth checking how full you fill the hole. A wider 12 in hole needs about 4 bags.
How big should a fence post hole be?
QUIKRETE suggests a diameter about 3 times the post width and a depth about 1/3 of the post's overall length, plus 6 in of gravel. Your local code, frost depth or fence plans may require more.
Fast-Setting or regular concrete mix?
QUIKRETE's Fast-Setting mix is poured dry into the hole and then soaked with water, and it sets in 20–40 minutes. Regular Concrete Mix is mixed first. Use the yield for the bag you buy.
Does this include gravel?
No. Enter only the depth that will be concrete. Gravel at the bottom and soil on top are not counted.
Sources
- QUIKRETE Fast-Setting Concrete Mix (No. 1004) product data sheet: a 50 lb bag yields approximately 0.375 cu ft; two 50 lb bags set a 4 in post in a 10 in hole 2 ft deep; fill to 3–4 in below the top (checked Oct 23, 2026)
- QUIKRETE Concrete Mix (No. 1101) product data sheet: yields per bag: 40 lb about 0.30 cu ft, 60 lb about 0.45 cu ft, 80 lb about 0.60 cu ft; for fence posts, dig the hole about 3 times the post diameter and 1/3 of the overall post height deep; follow local codes for load-bearing footings (checked Oct 23, 2026)
- QUIKRETE project guide: Setting posts: hole 3 times the post diameter, 1/3 of the post length deep plus 6 in of gravel for drainage; structural posts at least 6 in below the frost line (checked Oct 23, 2026)
- BEA: Regional Price Parities by State and Metro Area: state goods price levels used for the optional regional adjustment (checked Oct 22, 2026)
Sources last checked Oct 23, 2026Built and maintained by BJP.Written and reviewed by the Project Cost Math team against the sources above.How we calculate