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Fence Post Concrete Calculator — Postcrete & Ready-Mix
Calculate concrete needed for fence post holes. Get postcrete bag quantities or ready-mix volume for any number of posts.
Typical: 300mm
Trade rule of thumb: bury ≥⅓ of post length. ~600mm for a 1.8m fence, ~750mm for 2.0m, deeper on exposed sites (Tarmac Postcrete advises a hole ⅓ of the above-ground fence height)
Default: 20kg bags
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How We Calculate This
This calculator determines the concrete volume needed by calculating the volume of each post hole (cylinder) minus the volume of the post itself, then multiplying by the number of posts.
The formula
Concrete per hole = Hole volume (cylinder) - Post volume in hole
Total concrete = Concrete per hole × Number of posts
For a quick rule of thumb on how much postcrete you need per fence post, see our answer page.
Frequently Asked Questions
For a standard 300mm diameter × 600mm deep hole, you typically need 2 bags of 20kg postcrete for a standard fence, or 3 bags for fences 2m or taller. Larger holes or deeper settings need more. This calculator gives you the precise quantity.
Fence post holes should be at least 600mm deep for standard 1.8m panels. For taller fences or exposed sites, increase to 750mm. The hole should be about 300mm in diameter.
Postcrete (fast-setting post mix) sets in 5-10 minutes and is ideal for small jobs. For many posts, ready-mix concrete is more cost-effective. Both provide excellent support when properly mixed.
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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.