Post Hole Concrete Calculator
Work out how many concrete bags you need to fill post holes for fencing or decking from hole size and post diameter, free in your browser.
Enter Values
Minimum 0
Minimum 1
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the hole diameter and depth (mm).
- Optionally enter a square post size and the number of holes.
How it works
Hole volume = π × radius² × depth. If you enter a post size, its volume is subtracted from each hole.
Worked example
300 mm × 600 mm hole. ≈ 0.042 m³ of concrete per hole.
Common mistakes
- Forgetting the post displaces concrete — enter the post size to subtract it.
- Not ordering extra for spillage and uneven holes.
Frequently asked questions
How much concrete per post hole?
Hole volume minus the post's volume gives the concrete needed. The tool subtracts the post so you do not over-order.
How big should a post hole be?
A common guide is a hole about three times the post width and a depth of one-quarter to one-third of the post's above-ground height.
How many bags is that?
A 20 kg bag of pre-mix yields roughly 0.01 m³, so ~0.042 m³ is about 4 bags per hole — check the bag's stated yield.
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Cite this tool
Zerdly. (2026). Post Hole Concrete Calculator. Retrieved from https://www.zerdly.com/tools/post-hole-concrete-calculator/
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.