Shed Base Calculator
Works out the concrete for a shed base. Size the base to the shed footprint plus a small margin all round — most people pour it exactly to the shed size and then have nowhere to stand while assembling it.
Includes the waste allowance — this is the figure to order.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
volume = length × width × thickness × (1 + waste)
A slab is a rectangular prism, so the volume is the three dimensions multiplied together. The only judgement in it is the waste allowance — concrete cannot be topped up once it starts curing, so under-ordering is far more expensive than over-ordering.
What to watch out for
Three things specific to a shed base:
- Add 50–75 mm all round, not more. Enough to catch drips off the roof and to stand on while building; any wider and rain splashes back onto the cladding.
- Get it level and square before you pour. A shed is a rigid box — a base out of square by 20 mm will fight you at every panel.
- 100 mm is enough for a timber shed. Only go heavier if you are putting a lathe, a car lift or a kiln in it.
Frequently Asked Questions
How thick should a shed base be?
100 mm (4 in) over 100 mm of compacted hardcore handles any normal shed or summerhouse. A workshop with heavy machinery justifies 150 mm.
How much bigger than the shed should the base be?
50–75 mm beyond the footprint on each side. Enough to work on, not so much that rain splashes off the slab back onto the timber.
How much concrete for a 6 × 8 ft shed base?
About 0.19 m³ at 100 mm — a quarter of a cubic yard. That is well under any ready-mix minimum, so this is a bag job: roughly 16 × 25 kg bags.
Do I need a damp-proof membrane?
Under a shed base, yes — 1200-gauge polythene between the hardcore and the concrete. It keeps ground moisture out of the slab and out of the timber sitting on it.