Concrete around a post
(hole area − post area) × depth × countHole area is π × radius². Lumber presets subtract the actual rectangular post area; round and custom sizes use the entered outside diameter.
Concrete post-hole calculator
Estimate the concrete that fills a cylindrical hole after subtracting the space occupied by each post.
Enter hole and post sizes
The post volume is subtracted from each cylindrical hole.
Use actual hole dimensions
- Measure the hole across its widest usable diameter.
- Measure concrete depth, excluding gravel or soil backfill.
- Choose the nominal lumber size or enter the measured post width or outside diameter.
- Group holes by size when soil or drilling produces meaningful variation.
Worked example
Eight 12 in × 3 ft holes around nominal 4×4 posts
Cross-sectionπ × 0.50² − (3.5 ÷ 12)² = 0.700 ft²
Measured0.700 × 3 × 8 = 16.81 ft³
With 10% waste16.81 × 1.10 = 18.49 ft³
80 lb bags18.49 ÷ 0.60 = 30.81, rounded up to 31
One 12-inch hole around a nominal 4×4 post
| Concrete depth | Concrete volume | 80 lb bags at 0.60 ft³ |
|---|---|---|
| 2 ft | 1.40 ft³ | 3 bags |
| 3 ft | 2.10 ft³ | 4 bags |
| 4 ft | 2.80 ft³ | 5 bags |
This table is a material comparison, not a recommendation for hole diameter or embedment. Add an allowance for irregular excavation and verify the bag's stated yield.
Assumptions and limitations
- Holes are treated as straight cylinders with equal dimensions.
- The post extends through the full concrete depth.
- Bell bottoms, flared tops, collars, voids, and gravel are not included.
- The entered dimensions are quantities, not design recommendations.
Confirm the installation
Post embedment, hole size, drainage, uplift resistance, frost depth, bracing, concrete type, and cure time depend on the structure and site. Follow product instructions, approved plans, and local requirements.