Six 12-inch holes
Typical fence or small deck post layout.
24 bagsConcrete bagspost hole concrete calculator
Post Hole Concrete Calculator
A typical 12-inch diameter, 36-inch deep post hole around a 4-inch post needs about 1.9 cubic feet before waste.
Inputs
Results
Immediate estimateConcrete bags24 bags80 lb bags rounded up
Base concrete12.57 cu ft
Waste allowance1.26 cu ft
Adjusted concrete13.82 cu ft
Equivalent yards0.512 cu yd
Shopping list
- 24 bags 80 lb concrete mix
Diagram
Post hole volume
A cylindrical hole with the post volume subtracted before waste and bag rounding.
- Hole diameter
- Depth
- Post void
How this calculator works
Enter hole size, post size, count, and bag size to estimate how many concrete bags to buy.
Formula
The engine calculates cylindrical hole volume, subtracts the post volume, multiplies by the number of holes, adds waste, then rounds up by selected bag yield.
Field caution
Footing depth depends on frost depth, soil, load, and local requirements. This calculator estimates material quantity only.
Hole volume
π × radius² × depth × hole countPost displacement
π × post radius² × depth × post countPurchase quantity
(net volume + waste) ÷ bag yield, rounded upAssumptions
- Bag yield: 80 lb bag yields about 0.6 cu ft
- Shape: Cylindrical holes with a centered post void
Worked examples
Shallow test case
Shows the depth warning.
4 bagsConcrete bagsFAQ
Does this include gravel at the bottom of the hole?
No. Reduce the concrete depth if you plan to reserve part of the hole for compacted gravel.
Can I use it for Sonotube forms?
Yes, use the form diameter as the hole diameter and set post diameter to zero if the form is poured solid.
Sources and review
- Concrete Calculator, QUIKRETE. Accessed 2026-07-16.
- 2021 International Residential Code, International Code Council, 2021. Accessed 2026-07-16.
Reviewed 2026-07-16. Planning estimates only; confirm project-specific requirements.