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
80lb bags needed (total)
13
Net concrete per post
0.869
cu ft
Total cubic feet
6.95
Posts
8
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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 (in).
- 4Enter number of posts.
- 5Enter waste factor (%).
- 6Read your 80lb bags needed (total) on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
Setting fence posts in concrete means digging a round hole per post, dropping the post in, and filling the annular gap around it with concrete — and the volume needed per hole is the hole's cylindrical volume minus the volume the post itself displaces. This calculator takes your hole diameter and depth, your post's cross-section width (treated as displacing a square footprint, a reasonable approximation for both square and round posts), and your total post count, then computes cubic feet per hole, converts to 80lb bags using the standard 0.6 cubic feet per bag yield, and multiplies by the number of posts for a total order. Fence contractors typically use one to two 50lb 'fence post mix' quick-set bags per post for small residential posts, or standard concrete for larger gate posts and corner posts that carry more load — this tool works for either since you're specifying your own hole dimensions. It also adds a waste factor since post holes are rarely perfectly cylindrical and some concrete always spills at grade.
Worked example
Using the values the calculator loads with:
Inputs
- Hole diameter: 10 in
- Hole depth: 24 in
- Post width: 4 in
- Number of posts: 8
- Waste factor: 10 %
Results
- 80lb bags needed (total): 13
- Net concrete per post: 0.869
- Total cubic feet: 6.95
- Posts: 8
What each field means
Inputs
- Hole diameter (in)
- The hole diameter used in the calculation, measured in in. Starts at 10 in so you have a working example on load. Accepted range: 4–24 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: 6–48 in.
- Post width (in)
- The post width 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 8 so you have a working example on load. Accepted range: 1–200.
- Waste factor (%)
- The waste factor used in the calculation, measured in %. Starts at 10 % so you have a working example on load. Accepted range: 0–25 %.
Results
- 80lb bags needed (total)
- 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.
- Net concrete per post
- cu ft
- Total cubic feet
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Posts
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How deep should a fence post hole be?
A common rule is one-third of the post's above-ground height, or a regional frost-depth minimum, whichever is deeper — many residential fence posts use 24-30 inch holes for 6-8 foot fences.
Do I need to subtract the post's volume?
Yes, otherwise you'll over-order — the post physically displaces concrete in the hole, and for larger posts (4x4 or 6x6) that displacement is meaningful, especially in narrower holes.
Can I use quick-set concrete instead of standard mix?
Yes, quick-set (no-mix, pour-dry-then-water) concrete is popular for fence posts since it sets in 20-40 minutes without needing to hold the post plumb for hours, though it's typically sold in smaller 50lb bags.
Should every post use the same hole size?
No — gate posts and corner posts carrying more lateral load are commonly set in wider, deeper holes than standard line posts; run this calculator separately for each post type and add the results.
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
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60lb Concrete Bags Calculator
Bag count for jobs sized around the lighter, easier-to-carry 60lb mix.
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Concrete for Fence Posts Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/concrete-fence-post-calculator
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/concrete-fence-post-calculator" target="_blank" rel="noopener">Concrete for Fence Posts Calculator — RevenueLab</a> (2026).</p>
Source: [Concrete for Fence Posts Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/concrete-fence-post-calculator) (2026).