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 post mix (concrete) in the gap between the post and the sides of the hole — not the full volume of the hole itself, since the post displaces part of it. This is the figure that actually determines your bag count.
How the calculator accounts for post displacement
The calculator works out the gross hole volume (a cylinder for a round hole, a rectangular prism for a square one) and subtracts the volume of the buried post over the same depth. The remainder is what you need to buy — this is why a thicker post needs slightly less post mix than a thin one in the same hole.
How bag yield affects the answer
Post mix yield varies between products — a 20 kg bag from one manufacturer won’t necessarily cure to the same volume as another. Use the common bag size as a starting point, then swap in the yield printed on your own product for a tighter number.
Bags are rounded up per hole rather than for the whole job at once, so you don’t run short mixing one post at a time — it costs a little extra material on paper, but avoids a second trip to the merchant.
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 1.8–2.4 m for panel fencing. 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
Ten fence panels at 1.83 m (6 ft) each make an 18.3 m run, needing 11 posts at 1.83 m spacing. For a 250 mm diameter, 600 mm deep round hole with a 50 mm gravel base and a 75 mm square post:
- Concrete depth per hole: 550 mm (600 mm total depth minus the 50 mm gravel base)
- Post mix needed per hole, with 10% waste: about 24 litres
- Bags needed per hole (20 kg bag, ~8–9 litre yield): 4
- Total for the run: about 289 litres, or 44 bags
Why a gravel base
Adding gravel or hardcore before pouring isn’t just tradition — it stops the post base from sitting in trapped water, a leading cause of timber posts rotting from underground up. It also nudges the post mix volume down slightly, since the gravel takes up part of the depth that would otherwise need coverage.
Common bag sizes in the UK
Post mix (postcrete) is typically sold in 20 kg bags, yielding roughly 8–9 litres — this varies by manufacturer, so check the coverage printed on your bag rather than assuming a fixed figure.
Methodology
This tool works out material quantity — the volume of post mix implied by your hole and post dimensions, converted to bags using the yield you enter, plus a waste allowance. It doesn’t assess structural adequacy: hole depth, diameter, soil, fence height, and wind exposure all affect what a safe installation actually needs, and those are site-specific decisions. Choose suitable hole dimensions first, then use this calculator to plan materials.
FAQs
Is a postcrete calculator the same as this fence post concrete calculator?
Yes. "Postcrete" is the common name for fast-setting post-mix concrete used in fence posts (originally a product name that's become a general term, similar to how some brand names get used generically). Whether you call it a postcrete calculator, a fence post concrete calculator, or a post hole concrete calculator, the underlying job — working out volume and bag count — is the same.
How many bags of post mix per fence post?
For a typical 250 mm diameter, 600 mm deep hole with a 75 mm post, most 20 kg bags of post mix work out to around 3–4 bags per hole once waste is included. Deeper holes or wider posts need noticeably more, so it's worth checking your own numbers rather than assuming a flat rate per post.
Do I need hardcore or gravel under a fence post?
Many UK installers add 50 mm of gravel or hardcore at the bottom of the hole to help with drainage. It's optional in this calculator — set it to 0 if you're not using one.
How deep should a fence post hole be in the UK?
This calculator doesn't set hole depth for you — that depends on fence height, wind exposure, soil, and post type. A commonly cited minimum is around 600 mm for a standard garden fence, but always check manufacturer guidance or a local tradesperson for anything taller or more exposed.