Inputs to result
Concrete cu ft = count × π × (hole diameter² − post diameter²) ÷ 4 × depth ÷ 1,728; bags = ceil(concrete cu ft ÷ selected bag yield).
Concrete & Masonry
Post Hole Concrete Calculator
Estimate concrete volume and 40 to 80 lb bag counts for round post holes in feet and inches, automatically subtracting buried post displacement.
Enter hole diameter, buried depth, post diameter and hole count. The result subtracts the post's circular cross-section from the hole before estimating bags.
Calculation and worked example reviewed 2026-11-12. See our calculation methodology.
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Concrete & Masonry
US UNITSEnter your measurements
YOUR ESTIMATE
24 × 80 lb bags13.96 cu ft concrete0.52 cu yd across 10 holes.
Approximate 0.6 cu ft yield per bag; verify the selected product.
Formula and example
How this calculation works
This estimator converts your entered jobsite inputs (hole diameter, round post diameter, concrete depth, number of identical holes, concrete bag size) into consistent US trade units, computes the exact geometric baseline first, and then applies any selected waste or order allowance so you can verify both numbers before purchasing materials.
How does the post hole concrete calculator formula work?
Concrete cu ft = count × π × (hole diameter² − post diameter²) ÷ 4 × depth ÷ 1,728; bags = ceil(concrete cu ft ÷ selected bag yield).
(Post Hole Concrete Calculator)where:
- Hole diameter – Entered in in (default reference: 12 in);
- Round post diameter – Entered in in (default reference: 4 in);
- Concrete depth – Entered in in (default reference: 24 in);
- Number of identical holes – Entered in holes (default reference: 10 holes);
- Concrete bag size – Entered in selected option (default reference: 80 );
Check the math
Ten 12 in diameter × 24 in deep holes around 4 in round posts use about 13.96 cu ft of concrete, or 24 bags at 0.60 cu ft per 80 lb bag.
Quick lookup chart
Post Hole Concrete Volume & 80 lb Bag Guide (4 in Round Post Displacement)
Based on cylindrical holes with a 4 in round post subtracted and 0.60 cu ft yield per 80 lb bag.
| Hole Size (Dia × Depth) | Net Concrete / Hole | 1 Hole (80 lb Bags) | 10 Holes (80 lb Bags) |
|---|---|---|---|
| 10 in × 24 in | 1.00 cu ft | 2 bags | 17 bags |
| 12 in × 24 in | 1.40 cu ft | 3 bags | 24 bags |
| 12 in × 36 in | 2.09 cu ft | 4 bags | 35 bags |
| 12 in × 42 in | 2.44 cu ft | 5 bags | 41 bags |
Before you use this result
Assumes a round post centered in each cylindrical hole. Square posts, bell-shaped bases, uneven excavation and manufacturer-specific bag yields require a separate check.
- A general field rule of thumb is hole diameter = 3× post width, dug below the local frost line plus 4–6 in of tamped gravel drainage base.
- Set post diameter to 0 in if you are pouring solid cylindrical concrete piers (Sonotubes) with top-mounted post bases.
- Slope the top of the concrete collar slightly away from the post above grade so rainwater sheds instead of pooling against the wood.
Questions answered
Calculator FAQ
Does the calculator subtract the post?+
Yes, when a round post diameter is entered. Set it to zero for a solid cylindrical pier.
Why might the actual bag count differ?+
Hole shape, loose soil, depth variation and product-specific bag yield can change the quantity.
Method references: QUIKRETE Concrete Mix yield guidance