How many bags of concrete do I need for fence posts?
Measure the round hole, subtract the space occupied by the post, add the holes together, include a deliberate allowance, and round the bag count up.
Ten 12-inch holes at 36 inches deep
Actual 3.5-inch-square posts with 10% extra
The calculation
For one 12-inch-diameter hole, the round cross-section is π × 6² = 113.10 square inches. Subtract 3.5 × 3.5 = 12.25 square inches for the post, multiply by 36 inches of depth, and divide by 1,728:
Common post-hole examples
| Round hole | Depth | Net concrete* | 80-lb bags* |
|---|---|---|---|
| 8 in diameter | 24 in | 0.53 ft³ | 1 |
| 10 in diameter | 30 in | 1.15 ft³ | 2 |
| 12 in diameter | 36 in | 2.10 ft³ | 4 |
| 14 in diameter | 42 in | 3.44 ft³ | 6 |
*Per hole, using a 3.5-inch-square post, 0.60 ft³ yield per 80-lb bag, and no extra-material allowance. Verify the product label.
Four checks before buying
- Confirm the required hole: frost depth, wind exposure, fence height, soil, post type, and local rules can change its dimensions.
- Use actual post size: a nominal 4×4 commonly measures about 3.5×3.5 inches, but measure the material you have.
- Plan for irregular excavation: loose soil, bell-shaped bottoms, and over-digging can increase concrete volume.
- Read the bag yield: product type and bag size—not weight alone—control how much finished concrete one bag produces.
Frequently asked questions
Should every fence post be set in concrete?
That is a project and site decision. Fence system instructions, post material, soil, drainage, wind, frost, and local requirements should guide the installation method.
Should I subtract the post volume?
Subtracting it improves the geometric estimate. In practice, rough or oversized holes may use more concrete than that subtraction saves, which is why an extra-material allowance remains useful.
Can I mix all the bags at once?
Only with equipment, staffing, and placement timing appropriate to the product instructions. Plan manageable batches and follow the manufacturer’s water, mixing, placement, and curing guidance.