What this calculator works out
This calculator takes your base dimensions and the type of base you intend to build, and returns the area along with the materials that base needs — sub-base in every case, plus slabs, concrete or gravel depending on what you choose.
A shed base fails in one of two ways: it moves, or it holds water against the timber. Both are decided before the shed arrives.
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Before you build
- Make the base at least 50 mm larger than the shed on every side, so rain runs off the edge rather than tracking underneath.
- All three base types assume 100 mm of compacted hardcore sub-base underneath.
- The slab option assumes 600 by 600 mm paving on a 40 mm sharp sand bed.
- Keep the base level, with the ground around it falling away. A shed sitting in a dip will rot at the bearers.
Timber sheds rot from the bottom because the floor stays damp. Bearers laid on a solid base with no ventilation gap trap moisture; a gravel base or bearers raised on treated timber or plastic feet allow air to move and dry the underside. That single detail often decides whether a shed lasts ten years or twenty-five.
How this calculator works
An area, then materials by base type:
Area = length × width
Sub-base (all types) = area × 0.1 m³
Slabs: sand = area × 0.04 m³ · 600 mm slabs = area ÷ 0.36 × 1.08
Concrete: concrete = area × 0.1 m³ plus a damp-proof membrane
Gravel: gravel = area × 0.05 m³ plus a weed membraneAll three assume a 100 mm sub-base, which is the layer that stops the base settling. Gravel volumes are also shown as an approximate weight, since gravel is usually sold by the tonne rather than by volume.
Worked example: a 3 m by 2.4 m base
Using the default figures — a 3 m by 2.4 m slab base:
- Area: 7.2 m²
- Hardcore sub-base at 100 mm: 0.72 m³
- Sharp sand bedding at 40 mm: 0.29 m³
- 600 by 600 mm slabs including 8% waste: 22
Switching to concrete gives 0.72 m³ of concrete over the same sub-base; gravel gives 0.36 m³, about 576 kg. Gravel is the cheapest and drains best, concrete is the most permanent, and slabs sit between the two while being much easier to lift and relay if the ground settles.
Common mistakes
- Building the base the same size as the shed. Rain will run off the walls onto the edge and under the floor.
- Laying straight onto soil or grass. The base will sink unevenly within a season.
- Skipping the sub-base to save time. It is what stops the base moving.
- Placing bearers directly on solid concrete with no ventilation. Trapped damp rots the floor.
- Siting the shed at the bottom of a slope. Water collects there whatever the base is made of.
Frequently asked questions
Which base type is best?
Gravel over a compacted sub-base is cheap, drains well and is easy to correct if it settles — a good default for a garden shed. Paving slabs give a firm level surface and can be lifted and relaid. Concrete is the most permanent and the most work, and is worth it for a large or heavy building such as a workshop with machinery.
Do I need planning permission for a shed?
Usually not, under permitted development, provided it is single storey, not forward of the principal elevation, within height limits (typically 2.5 m within 2 m of a boundary), and the total outbuilding coverage stays within limits. Listed buildings, conservation areas and some new-build estate covenants have additional restrictions. Check with your local authority if in doubt.
How do I make sure it is level?
Set out the perimeter with pegs and string, check across both diagonals to confirm it is square, and use a long spirit level on a straight edge across the base in several directions. Compacting the sub-base in layers rather than all at once is what keeps it level afterwards.
Should the base be perfectly flat or have a fall?
Flat and level for the base itself, so the shed sits square and doors close properly. The ground around it should fall away so water drains off rather than pooling at the edges. That is a landscaping question rather than a base one, and it is worth resolving before building.
Is what I enter stored?
No. Dimensions are processed in your browser and never transmitted or retained.
Related tools
References
- GOV.UK — permitted development rules for outbuildings and garden buildings
- Health and Safety Executive — manual handling of slabs and aggregates, and cement safety
Sources are checked at publication and can change — how I choose and check references.