Post Hole Concrete Calculator
Enter your hole size, post size, and number of posts to estimate the concrete volume and the number of bags needed to set your posts.
Results
- Bags per hole
- 2 bags
- Total bags
- 9 bags
- Concrete per hole
- 0.87 cu ft
- Total concrete
- 5.21 cu ft
Estimates only. Round up and buy a little extra to account for waste, spills, and breakage.
This calculator works out the concrete needed per post hole and across the whole run, reporting cubic feet, cubic yards and 50 or 80 lb bag counts.
It subtracts the post volume from the hole so you are not paying for concrete that the post itself occupies.
Also works as a fence post concrete calculator, post hole bag calculator, and deck footing estimator.
How to use the post hole concrete calculator
- Enter the number of posts.
- Enter the hole diameter in in.
- Enter the hole depth in in.
- Enter the post width in in.
- Enter the bag size.
- Optionally add a price to estimate the total project cost.
- Read your quantities instantly — no sign-up, and you can print the estimate to take to the supplier.
Worked example
Inputs
- Number of posts
- 6
- Hole diameter
- 10 in
- Hole depth
- 24 in
- Post width
- 4 in
- Bag size
- 80 lb bag (0.60 cu ft)
Results
- Bags per hole
- 2 bags
- Total bags
- 9 bags
- Concrete per hole
- 0.87 cu ft
- Total concrete
- 5.21 cu ft
Using the example values above, the post hole concrete calculator returns 2 bags (bags per hole). Change any field to match your own project and the numbers update instantly.
Hole sizing rules of thumb
Dig the hole three times the post width and one third of the post height deep. A 4 by 4 post standing 6 feet above grade wants a 12 inch diameter hole about 2 feet deep.
Always go below your local frost line, otherwise frost heave will lift the post over the first winter.
Bags per hole
A 12 inch diameter hole 2 feet deep holds about 1.5 cubic feet after the post displaces its share, which is 2 bags of 80 lb mix or 3 bags of 50 lb fast setting mix.
Add 3 to 4 inches of gravel at the bottom of each hole for drainage, then crown the concrete above grade so water runs away from the post.
Concrete bag yields
| Bag size | Yield (cu ft) | Bags per cu yd |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 50 lb | 0.375 | 72 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
Formula
Hole volume = π × (diameter/2)² × depth. Subtract post volume. Bags = concrete volume ÷ bag yield, per hole × posts.
Frequently asked questions
- How many bags of concrete per fence post?
- A typical 4×4 post in a 10-inch hole 24 inches deep needs about 1–2 bags of 80 lb concrete. Wider or deeper holes need more.
- How deep should a post hole be?
- Set posts at least one-third of the post's above-ground height deep, and below your local frost line — commonly 24–36 inches.
- Do I need gravel at the bottom of a post hole?
- Yes — add 4–6 inches of gravel for drainage so water doesn't pool around the post base and rot or heave it.
- How many bags of concrete per fence post?
- Usually 1 to 2 bags of 80 lb mix for a 4 by 4 post in a 10 to 12 inch hole two feet deep.
- Can I pour dry concrete in a post hole?
- Fast setting mix is designed for it: fill the hole, add the listed amount of water and let it cure. For structural posts, mixing first still gives more reliable strength.
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