Post Hole Concrete Calculator
Estimate bags of concrete per fence or deck post hole, subtracting the post itself.
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What this tool does
Estimate bags of concrete per fence or deck post hole, subtracting the post itself. Hole volume is a cylinder minus the buried post section. Bell-shaped or rocky holes use more; the waste allowance covers irregular digging.
How to use the Post Hole Concrete Calculator
- Enter or select hole diameter in in.
- Enter or select hole depth in in.
- Enter or select post size (square) in in.
- Enter or select number of posts.
- Enter or select waste allowance (%).
- Enter or select bag weight.
- Read the calculated result; change any measurement to compare alternatives.
Formula
per hole cu ft = π×(diameter/24)²×(depth/12) − (post/12)²×(depth/12); bags = ceil(total cu ft × (1 + waste/100) / bag yield)
- diameter
- Hole diameter (in, before metric conversion)
- depth
- Hole depth (in, before metric conversion)
- post
- Post size (square) (in, before metric conversion)
- count
- Number of posts
- waste
- Waste allowance (%)
- bag
- Bag weight
Hole volume is a cylinder minus the buried post section. Bell-shaped or rocky holes use more; the waste allowance covers irregular digging.
Worked example
For post hole concrete calculator, the following measurements illustrate the exact method: Hole diameter: 12; Hole depth: 36; Post size (square): 3.5; Number of posts: 10; Waste allowance (%): 10; Bag weight: 0.3.
Inputs
- Hole diameter12 in
- Hole depth36 in
- Post size (square)3.5 in
- Number of posts10
- Waste allowance (%)10
- Bag weight40 lb (0.3 cu ft yield)
Result
- Bags to buy78
- Concrete per hole2.1 cu ft
- Total concrete23.11 cu ft
- Total volume0.86 cu yd
Typical concrete bag yield
| Bag weight (lb) | Yield (cu ft) |
|---|---|
| 40 | 0.3 |
| 60 | 0.45 |
| 80 | 0.6 |
Results explained
- Bags to buy
- Bags to buy from the formula above. Hole volume is a cylinder minus the buried post section. Bell-shaped or rocky holes use more; the waste allowance covers irregular digging.
- Concrete per hole
- Concrete per hole from the formula above, expressed in cuft before metric conversion. Hole volume is a cylinder minus the buried post section. Bell-shaped or rocky holes use more; the waste allowance covers irregular digging.
- Total concrete
- Total concrete from the formula above, expressed in cuft before metric conversion. Hole volume is a cylinder minus the buried post section. Bell-shaped or rocky holes use more; the waste allowance covers irregular digging.
- Total volume
- Total volume from the formula above, expressed in yd3 before metric conversion. Hole volume is a cylinder minus the buried post section. Bell-shaped or rocky holes use more; the waste allowance covers irregular digging.
Frequently asked questions
A 12 in diameter × 36 in deep hole with a 4×4 post needs about 2.2 cu ft — roughly four 80 lb bags (0.6 cu ft each) per post before waste.
A common guide is 3× the post width in diameter (12 in for a 4×4) and depth below the frost line, often ⅓ of the post length plus 6 in for a gravel base.
Yes — the post displaces concrete. This tool subtracts the buried square section, which is why its bag count is lower than cylinder-only calculators.
A few inches of compacted gravel under the post improves drainage and helps the post resist frost heave; it slightly reduces the concrete needed.
Some fast-set products are rated for dry placement with water added in the hole. Follow the bag instructions — structural deck posts deserve a proper wet mix.