Calculate how much concrete you need for post holes, shed bases and small slabs — from bags of post mix and ready-mix to larger volumetric jobs.
Enter the area and depth for slabs, or count your post holes. Post mix sets in 5–10 minutes with no mixing — one bag per standard fence post hole.
01 — The Calculator
Pick a material, pace out your plot, and the value engine prices every way to fill it — hippo bags, dumpy bags or handy sacks.
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10 cm
Depth changes the volume a lot — 5cm mulch is normal, raised veg beds want 20cm+.
Not sure? See the depth guide belowJust-add-water bags — the easy option for posts, small pads and repairs.
Worked example: fencing a 20m run
12 posts (11 bays plus ends) needs 12 bags of post mix. A small 2m² shed base at 10cm deep is 200 litres — about 10–12 bags of ready-mix concrete.
A 25kg bag of concrete mix yields about 12.5 litres, so a cubic metre takes roughly 80 bags — past half a cubic metre, a delivered ready-mix load usually beats bags on price and effort. For shed bases, 75–100mm over compacted hardcore is right; driveways and garages need 100–150mm with mesh. Mixing your own from ballast, use about a 1:5 cement-to-ballast ratio for general slabs and footings.
Concrete weighs around 2.4 tonnes per cubic metre — plan barrow routes before the lorry arrives, and have enough hands: you typically get 60–90 minutes of working time per mix. Cast onto a damp-proof membrane, tamp level to a slight fall, and keep it damp while it cures for at least 48 hours (a week before heavy loads). Avoid casting in frost or on days over 25°C — both wreck the cure. For fence posts, rapid post-mix needs no mixing at all: one 20kg bag per post in normal ground.
One 20kg bag per standard hole (about 200×200×600mm). Use two bags for gate posts or exposed, windy sites.
Last reviewed: October 2026 · Reviewed by the Garden Calculator editorial team.
Garden Calculator — Project
My garden project · 28 October 2026
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