How This Calculator Works
Post holes are repetitive, so small rounding errors multiply fast. This calculator estimates concrete around square posts using hole diameter, hole depth, post width, and post count.
Formula
Concrete per hole = cylinder volume of the hole minus square post displacement. Total volume = per-hole volume x number of holes x overage.
Assumptions
The calculator assumes straight-sided holes. Bell-shaped footings, loose soil, frost-depth requirements, and gravel drainage bases may change the quantity.
Measurement Checklist
- Measure hole diameter after digging because loose soil can widen holes beyond the auger size.
- Subtract the post width only for the portion of the post that sits inside concrete.
- Reduce concrete depth if gravel is placed at the bottom of each hole.
- Use the same hole depth only when the grade and post design are consistent across the line.
Practical Examples
- Eight 10 inch diameter holes, 36 inches deep, around 4x4 posts with 10% overage need about 12.15 ft^3 of concrete, or 21 bags of 80 lb mix.
- Six 8 inch diameter holes, 24 inches deep, around 4x4 posts need about 3.5 ft^3 with 10% overage, or about 6 bags of 80 lb mix.
- Twelve 12 inch diameter holes, 30 inches deep, around 4x4 posts need about 23.1 ft^3 with 10% overage, or about 39 bags of 80 lb mix.
- A single mailbox post in a 10 inch diameter, 24 inch deep hole usually takes about 2 bags of 80 lb mix after allowing for overage.