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How the shed base calculator works
A shed base goes down in layers, and each layer is sized at a different density or mix, so this calculator works them out from the figures the site's other pages already use rather than inventing a second number for the same material. The compacted MOT Type 1 sub-base is weighted at 1.8 tonnes per cubic metre — the figure the gravel calculator and the paving calculator both use. The concrete mix, and the cement, sand and aggregate it takes, are the concrete calculator's own, so a shed base and a slab priced anywhere else on the site agree.
Start with the footprint. The base is the shed's length × width, grown by whatever overhang you want on each side — a base that runs a little proud sheds water away from the frame. Then the sub-base is worked out from its depth, and the top layer from its own figures: a concrete slab from its depth and mix, or paving slabs and their sand bedding, set out on the joint module the paving calculator uses.
What each layer is for
The sub-base is the layer that is easy to skip and expensive to skip. It spreads the shed's weight over a wider area and gives the slab or the paving a stable platform, so the base does not settle at one corner and twist the frame. That is why it is compacted in layers and why its depth matters as much as the slab's.
The top layer takes the wear. A concrete slab is stronger and is the usual choice for a workshop or anything heavy, and it can be laid over a damp-proof membrane if the floor has to stay dry. Paving slabs over a sand bedding are quicker, cheaper and easy to lift and relay if the ground moves under them — which is exactly what suits a light garden shed, and exactly why they are less suitable for anything heavy.
What this page does not do
This is a quantity estimate. It is not a structural design, and it does not check the ground, the drainage, the slope or the load the shed will carry. A small shed on firm, level, well-drained ground is one thing. A large shed, a workshop, a log store or anything with a heavy load needs a base designed for that load and for the ground it sits on — if the ground is soft, sloping, or made up, or if the shed is large or heavy, take advice from a competent person before you dig.
It also does not decide whether you are allowed to build. A small garden shed is often permitted development in England, but the limits depend on its size, its height, where it sits near a boundary and whether the property still has its permitted development rights. That is a planning question rather than a quantity one — the site's planning permission page is a starting point, not a decision. If in doubt, ask your local planning authority before you build.
Shed base FAQs
How thick should a shed base be?
For a light timber shed on firm, well-drained ground, 100 mm of concrete over a compacted sub-base is a common starting point, and that is the depth this calculator uses as a planning figure. A larger shed, a heavier load, or softer ground needs more — and the sub-base under it matters as much as the slab. This page works out quantity, not thickness: for anything beyond a light shed, get the depth from a competent person.
Do I need a sub-base under a shed base?
Yes, for a concrete slab. The slab should be poured over a compacted layer of MOT Type 1, not straight onto soil, because the sub-base spreads the load and gives a stable platform that will not settle unevenly. This calculator sizes that sub-base at 1.8 t/m³ compacted — the same figure the gravel and paving calculators use — over a 100 mm depth, which is the usual starting point.
Concrete slab or paving slabs for a shed base?
Both work. A concrete slab is stronger and handles a heavier shed, and it is the usual choice for a workshop or anything with a heavy load. Paving slabs over a sand bedding on a compacted sub-base are quicker, cheaper and easy to lift and relay if the ground moves, and they suit a light garden shed. Whichever you pick, the sub-base underneath is what stops it moving.
Does a shed need planning permission?
A small garden shed is often permitted development in England, but not always, and the rules turn on its size, its height, where it sits in relation to the boundary and whether it is at the front of the house. There are also cases where permitted development rights have been removed. Check with your local planning authority before you build — the site's planning permission page is a starting point, not a decision.
How much overhang should a shed base have?
A base that runs a little proud of the shed is easier to build and sheds water away from the frame, so 25 to 50 mm each side is common. It is a choice, not a rule, and this calculator treats it as a box: set it to 0 for a base exactly the size of the shed, or to the overhang you want and the area, the sub-base and the concrete all grow to match.