Post Hole Concrete Calculator
Post Hole Concrete Calculator concrete for post holes how many bags per fence post round footing volumeEstimate fill around equal square posts
Enter hole count, round-hole diameter, filled depth, square post width, extra allowance, and the bag yield printed on the product. The tool subtracts the post volume and rounds bag count up.
Volume around the post
Each hole is modeled as a cylinder. Its volume is π times radius squared times filled depth. The post is modeled as a square prism occupying post width squared times the same depth. Subtracting the post prism leaves the approximate concrete space.
Worked example
Ten holes measuring 12 inches across and 30 inches deep, each holding a 3.5-inch square post, need about 17.51 cubic feet before allowance. Ten percent extra raises that to about 19.26 cubic feet. If one bag yields 0.6 cubic foot, round 32.1 up to 33 bags.
Use the product's actual yield
Bag weight does not directly state placed volume. Mix formulations and package sizes vary, so read the labeled yield for the exact product. Water content, spillage, uneven excavation, voids, and ground absorption can change field usage. A whole-bag result is a purchasing estimate, not a promise.
Measure irregular holes conservatively
Hand-dug holes are rarely perfect cylinders. A flared bottom, crumbling sides, rocks, bell footing, gravel base, or different filled depth changes volume. Measure representative diameters and separate groups when holes differ materially rather than averaging a highly uneven project into one misleading number.
Design inputs come from elsewhere
This calculator does not choose diameter, depth, post size, concrete type, or whether concrete should be used. Frost depth, soil, drainage, wind, loads, structure, product instructions, local code, and utility locations all matter. Obtain appropriate project guidance before entering dimensions.
Limitations and safety
The model assumes equal round holes and centered equal square posts through the full entered fill depth. It excludes gravel, forms, above-grade collars, reinforcement, cure conditions, mixing capacity, and leftover partial bags. Follow safe digging, utility-location, lifting, and concrete-handling practices.
Post Hole Concrete FAQ
Why subtract the post?
The post occupies part of the cylindrical hole, leaving less space for concrete.
Can I use a round post?
Not directly. This version subtracts a square post cross-section.
Does this choose footing depth?
No. Depth is a visitor-supplied project input.