How much concrete for a shed base?
Cast a solid base for a shed, summerhouse or garden store — enter the footprint and we add a sensible overhang, then give you the concrete as ready-mix bags or as cement and ballast, plus the compacted hardcore underneath. Free, no sign-up.
A guide, not a guarantee. Ready-mix bags assume a yield of about 0.011 m³ per 25 kg bag (scaled to your bag size); cement-and-ballast assumes a roughly 1:5 mix at about 7.6 × 25 kg cement bags and 1.7 tonnes of ballast per m³. Hardcore assumes MOT Type 1 at about 2 tonnes per m³. Add a damp-proof membrane and reinforcing mesh for anything but the smallest base. Above roughly half a cubic metre, ordering ready-mixed delivered is usually cheaper and far less work than bags. Nothing you type leaves your browser.
Footprint, overhang, depth and mix.
Make the base bigger than the shed. A base a touch larger than the footprint lets rain drip clear of the timber instead of running back under it. We add your overhang to each side — 50 mm is typical — to get the base size.
Work out the volume. Base area times the concrete depth gives the volume. 100 mm suits most sheds; a very light store can go to 75 mm. We add about 10% wastage for spillage and an uneven dig.
Bags or bulk. For a small base, bagged ready-mix is easiest — we assume each 25 kg bag yields about 0.011 m³ and scale to your bag size. Mixing your own from cement and ballast is cheaper in bulk: we use a roughly 1:5 mix, about 7.6 bags of cement and 1.7 tonnes of ballast per cubic metre.
Don't forget the hardcore. Concrete sits on a compacted MOT Type 1 sub-base — about 75 mm — which we size in tonnes and bulk bags. Lay a damp-proof membrane over it before you pour, and add reinforcing mesh for anything but the smallest base.
When to order ready-mixed. Above roughly half a cubic metre, hand-mixing dozens of bags is hard going — a ready-mix delivery (or a mix-on-site lorry for smaller loads) is usually cheaper and far quicker. We'll flag it when you cross that line.
Keep every garden project in one place.
Stead remembers your outbuildings, their measurements and when each job was done — so the next project starts with the numbers already to hand.