Concrete Post Hole Calculator
Calculate multiple equal cylindrical post holes and subtract the portion occupied by round posts. The result includes cubic feet, cubic yards, and approximate bags.
What does the concrete post hole calculator calculate?
Find each hole’s cylindrical volume using π times radius squared times depth, subtract the embedded post’s cylindrical volume, and multiply by the number of holes. Add extra for soil variation and spillage. Hole dimensions, embedment, drainage, frost protection, and post design require local verification.
Concrete around round posts
The formula subtracts a round post cylinder from each round hole cylinder. Dimensions are converted to feet before volume is calculated, so the result is cubic feet and can then be converted to bags or yards.
How to measure for an accurate estimate
- Measure hole diameter across the widest point and depth from finished grade to the bottom.
- Check several holes because augers and loose soil rarely produce identical cylinders.
- Enter the embedded diameter of a round post, or zero to ignore displacement.
- Calculate bell-shaped bases, footings, or rectangular posts separately for a closer estimate.
Eight fence posts
Eight 12-inch-diameter holes are 3 ft deep around 4-inch round posts, with 10% extra.
Net volume is 16.76 cu ft; with extra it is 18.43 cu ft, or about 31 80-lb bags.Before you purchase material
- Call the required utility-location service before digging and respect marked exclusion zones.
- Confirm post depth, diameter, frost, drainage, uplift, and inspection rules locally.
- Keep bagged mix dry and compare total mixing effort with a small ready-mix delivery.
- Brace posts before placement and follow the concrete and post manufacturers’ curing guidance.
Common questions
How many bags of concrete do I need per fence post?
A 12-inch-diameter, 3-foot-deep cylindrical hole around a 4-inch round post uses about 2.10 cubic feet before waste, or roughly four 80-pound bags at 0.60 cubic foot each. Actual irregular holes may use more.
Should I subtract the fence post from the hole volume?
Yes, for a tighter material estimate, subtract the embedded post volume. The difference is most noticeable with large posts or many holes. For irregular or rectangular posts, the calculator’s round-post assumption is approximate, so retain a practical allowance.
How deep should a post hole be?
Depth depends on fence or deck loads, post height, soil, frost depth, wind, uplift, and local code. Do not choose depth from a concrete estimate. Follow approved plans, manufacturer instructions, and requirements from the local building authority.
Can I pour dry concrete mix into a post hole?
Only use a dry-placement method if the specific manufacturer expressly permits it and follow its water and installation directions. Conventional mixing generally provides better control. Structural posts need the specified concrete quality, consolidation, positioning, and curing.
Sources and reference standards
- QUIKRETE Concrete Mix Product DataManufacturer product data used to cross-check typical bag yields; verify the yield printed on the bag you buy.
- International Code Council — 2024 International Residential CodePrimary model-code source for residential foundations, reinforcement, masonry, and stair safety requirements.