Fence Post Concrete Calculator
Work out how much concrete and how many bags your fence posts need, with post displacement, a gravel base, and waste already accounted for.
- Concrete depth (per hole)
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- Hole volume (per hole)
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- Post displacement (per hole)
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- Net concrete (per hole)
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- Total concrete (with waste)
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- Bags needed
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- Estimated materials cost
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Not a structural specification
Planning the full fence project?
This estimate covers concrete for the holes you entered. Add your location, fence type, terrain, and installation preference for a full project-specific cost range.
Plan My Full Fence ProjectHow much concrete goes around a fence post?
A fence post only needs concrete in the gap between the post and the sides of the hole — not the hole’s full volume, since the post itself takes up part of that space. Getting the volume of concrete for fence posts right (rather than the full hole volume) is what keeps this calculator from overstating your bag count.
How the calculator accounts for post displacement
Picture the post sitting inside the hole: the concrete only fills the gap around it, not the whole cavity. The calculator works this out precisely — cylinder or prism volume for the hole, minus the post’s own volume over that same depth.
How bag yield affects the answer
Bag yield varies between products, even at the same weight — a 20 kg bag of one brand’s rapid-set mix won’t necessarily cure to the same volume as another. Use a common bag size as a starting point, then swap in your product’s stated yield for a tighter number.
Bags are rounded up per hole, not once for the whole job, so you won’t come up short mixing one post at a time — it costs a little extra material on paper, in exchange for not running out mid-pour.
Fence run length to post count
If you know your total run but not your post count, enter the fence length and your target post spacing (commonly around 2.4 m for paling fences). The calculator places a post at each end and at every full span, rounding up so the final gap is never longer than your spacing.
Worked example
A 20 m paling fence run with posts every 2.4 m needs 10 posts. For a 300 mm diameter, 600 mm deep round hole with a 50 mm gravel base and a 90 mm square post:
- Concrete depth per hole: 550 mm (600 mm total depth minus the 50 mm gravel base)
- Concrete needed per hole, with 10% waste: about 34 litres
- Bags needed per hole (20 kg bag, ~9 litre yield): 5
- Total for the run: about 379 litres, or 50 bags
Why a gravel base
Post holes in Australia often get a gravel base for the same reason plumbers care about drainage anywhere: standing water at the bottom of the hole is what eventually rots a timber post or corrodes a steel one. As a bonus, that gravel layer also trims the concrete volume, since it occupies part of the hole depth you’d otherwise fill.
Common bag sizes in Australia
Rapid-set post mix and general-purpose concrete are both commonly sold in 20 kg bags in Australia, yielding roughly 8–9 litres depending on the product — always check the yield printed on your specific bag rather than assuming a fixed figure.
Methodology
Material quantity is what this tool calculates — the volume of concrete implied by your hole and post dimensions, converted to bags using the yield you enter, plus a waste allowance. Structural adequacy isn’t covered: hole depth, diameter, soil, fence height, and wind exposure all affect what a safe installation actually needs, and those are site-specific decisions best confirmed with a local supplier or tradesperson. Choose suitable hole dimensions first, then use this calculator to plan materials.
FAQs
Is this the same as a post hole concrete calculator?
Yes — in Australia you'll see this searched as both a fence post concrete calculator and a post hole concrete calculator. Either way, you're working out bags of rapid-set or standard concrete per hole, which is exactly what this tool does.
How much concrete per fence post in Australia?
For a common 300 mm diameter, 600 mm deep hole with a 90 mm treated pine post, most 20 kg rapid-set bags work out to around 4–5 bags per hole once waste is included. Taller paling fences or exposed, windy sites often call for larger holes, so check your own numbers rather than assuming a flat rate.
Do I need to use rapid-set concrete for fence posts?
Rapid-set (fast-setting) post mix is common in Australia because it lets you brace and move on the same day, but standard concrete works too if you can wait for it to cure before loading the post. This calculator works with either — enter whichever product's yield you're using.
What size post hole do I need for a paling fence?
This calculator doesn't set hole size for you — it depends on fence height, wind exposure, post type, and soil. A commonly cited approach is a hole diameter roughly 3 times the post width and a depth around a third of the post's total length, but check manufacturer guidance or a local supplier for anything unusually tall or exposed.