Post Hole Concrete Calculator
Estimate concrete volume and whole 60- or 80-pound bag counts for round fence post holes, with the post displacement removed.
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How this calculator works
Concrete per hole = (π × hole radius² − post width²) × depth. All dimensions are converted from inches to cubic feet, multiplied by the number of holes, and increased by the selected extra-material percentage. Bag counts are always rounded up.
Worked example
Ten 12-inch-diameter holes, each 36 inches deep around an actual 3.5-inch-square post, require about 23.11 cubic feet after adding 10% extra. That rounds up to 39 80-pound bags or 52 60-pound bags.
Measure the hole, not just the post
Concrete volume is controlled by the hole diameter and depth minus the volume occupied by the post. Measure or specify the finished hole dimensions for each post type. Corner, gate, and structural posts may need larger footings than ordinary line posts, so calculate those groups separately rather than using one average for the entire job.
Soil, frost depth, wind exposure, post material, drainage, and local code can change the required footing. A calculator can convert dimensions to volume, but it cannot approve the footing design.
Plan the pour
Round bag quantities up and keep the product yield visible on the bag. Dry soil, oversized excavations, spillage, and uneven bottoms increase consumption. If the total approaches a substantial fraction of a cubic yard, compare bagged mix with ready-mix delivery while accounting for minimum-load and short-load fees.
Sources & assumptions
Round-hole volume uses standard cylinder geometry and subtracts the entered square post cross-section. Bag yields use QUIKRETE Concrete Mix product data. Actual excavation shape and product yield control the final order.
Frequently asked questions
Why subtract the post volume?
The post occupies space inside the hole, so the net concrete volume is smaller than a completely filled cylinder. Irregular holes can offset that difference.
Is a 12-inch-wide, 36-inch-deep hole always correct?
No. Post type, fence height, wind exposure, soil, frost depth, and local rules can require different dimensions. This tool calculates quantity from the dimensions you choose; it does not design the footing.
Should I buy 60- or 80-pound bags?
Choose based on handling, mixer capacity, availability, and the exact yield printed on the product. The calculator displays both common sizes.