Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Total bags needed
2
Concrete per post
0.48
Bags per post
1.06
Total concrete volume (cu ft)
0.48
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter hole diameter (in).
- 2Enter hole depth (in).
- 3Enter post width (square post) (in).
- 4Enter number of posts.
- 5Enter bag size.
- 6Read your total bags needed on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
Fence post concrete volume depends on the hole diameter, hole depth, and how much of that volume the post itself displaces. The rule of thumb is to dig the hole 3x the post's width in diameter and set it about a third of the total post length (or below the frost line) into the ground. This tool computes the hole's cylindrical volume, subtracts the post's own volume, and converts the remaining concrete volume into cubic feet and the number of standard 50 lb or 80 lb bags you need per post and for your whole run of fence.
Worked example
Using the values the calculator loads with:
Inputs
- Hole diameter: 8 in
- Hole depth: 24 in
- Post width (square post): 4 in
- Number of posts: 1
- Bag size: 60 lb bag (~0.45 cu ft)
Results
- Total bags needed: 2
- Concrete per post: 0.48
- Bags per post: 1.06
- Total concrete volume (cu ft): 0.48
What each field means
Inputs
- Hole diameter (in)
- The hole diameter used in the calculation, measured in in. Starts at 8 in so you have a working example on load. Accepted range: 4–36 in.
- Hole depth (in)
- The hole depth used in the calculation, measured in in. Starts at 24 in so you have a working example on load. Accepted range: 6–72 in.
- Post width (square post) (in)
- The post width (square post) used in the calculation, measured in in. Starts at 4 in so you have a working example on load. Accepted range: 1–12 in.
- Number of posts
- The number of posts used in the calculation. Starts at 1 so you have a working example on load. Accepted range: 1–200.
- Bag size
- Pick the option that matches your situation — the maths changes per option. Choices: 50 lb bag (~0.375 cu ft), 60 lb bag (~0.45 cu ft), 80 lb bag (~0.6 cu ft).
Results
- Total bags needed
- Returned as a whole number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Concrete per post
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Bags per post
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Total concrete volume (cu ft)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How much concrete do I need for a fence post?
For a typical 8-inch-diameter, 24-inch-deep hole with a 4x4 post, you need about 1.4 cubic feet of concrete per post after subtracting the post's volume — roughly one 60 lb bag (0.45 cu ft yield rounds up to about 3 bags per 4 posts, or just over one 50 lb bag each).
Why subtract the post's volume?
The post occupies space inside the hole that concrete would otherwise fill. For thick posts (6x6) this can shave 10-15% off the concrete needed versus assuming the hole is empty.
How deep should a fence post hole be?
General guidance is one-third of the post's above-ground height, or below your local frost line, whichever is deeper — commonly 24-36 inches for a 6-foot fence.
Should I round up on bags?
Always round up to the next full bag per post and buy a couple extra for spillage and uneven hole bottoms — running short mid-pour on quick-set concrete is a bigger problem than a leftover bag.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). How Much Concrete Do I Need for a Fence Post?. Retrieved from https://www.revenuelab.fyi/toolbox/how-much-concrete-for-fence-post
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/how-much-concrete-for-fence-post" target="_blank" rel="noopener">How Much Concrete Do I Need for a Fence Post? — RevenueLab</a> (2026).</p>
Source: [How Much Concrete Do I Need for a Fence Post? — RevenueLab](https://www.revenuelab.fyi/toolbox/how-much-concrete-for-fence-post) (2026).