Concrete calculator
Post Hole Concrete Calculator
Enter the filled concrete depth, hole shape and size, number of holes, and actual post dimensions. The calculator subtracts the embedded post volume and rounds the selected bag size up to whole bags.
Planning estimate, not a contractor quote. Review the assumptions and replace presets with local prices when available.
Method
How this calculator works
Post-hole concrete fills the space between the excavation and the embedded post. A round hole is calculated as a cylinder; a square hole is a rectangular prism. If a post is embedded through the full concrete depth, its actual cross-section can be subtracted before the allowance is added.
Concrete volume = (hole cross-sectional area − post cross-sectional area) × filled concrete depth × hole count. Adjusted volume = concrete volume × (1 + allowance percentage). Bags = adjusted cubic feet ÷ nominal bag yield, rounded up.
Worked example
A realistic starting point
One 12 in round hole filled 24 in deep around an actual 4 in by 4 in post contains 1.35 ft³ of concrete before allowance. With 10% added, the estimate is 1.48 ft³, which rounds to 4 nominal 50 lb bags.
What changes the answer
Factors worth checking
Filled depth, not excavation depth
Exclude any gravel drainage layer and the soil left above the concrete. Enter only the height that will actually contain concrete.
Use actual post dimensions
Nominal lumber dimensions may differ from measured dimensions. Subtract the section that is actually embedded in the concrete.
Irregular holes use more
Augered and hand-dug holes can flare, cave in, or vary in diameter. Increase the allowance when the excavation is not uniform.
Product yield controls bags
Bag yields vary by product and package size. Confirm the yield printed on the selected mix before purchase.
Common questions
Post-hole concrete FAQ
Should I subtract the post from the hole volume?
Yes when the post is embedded through the concrete. Select the post shape and enter its actual dimensions. Select ‘Do not subtract post’ for an anchor set into the top of the concrete.
How deep should a post hole be?
Depth depends on above-ground height, loads, soil, frost depth, and local requirements. Use the depth specified for the project; the calculator does not design the foundation.
How wide should the hole be?
QUIKRETE’s general post-setting instructions use a hole about three times the post width, but structural posts and local conditions can require a different size. Follow the project design and local rules.
Why does the calculator round up bags?
Concrete mix is sold in whole bags, so any partial calculated bag requires another package. Yield is nominal and field conditions can increase usage.
Evidence
Sources & review
- QUIKRETE — Setting Posts in ConcreteManufacturer instructions for general hole proportions, gravel, and fill level.
- QUIKRETE Concrete CalculatorManufacturer bag calculator and whole-bag rounding reference.
- QUIKRETE Concrete Mix Data SheetNominal packaged-concrete yields.
Published by Calcstead. Report a correction.
Method and source links reviewed September 4, 2026. Price presets are editable planning inputs and should be replaced with current local information.