Use this guide to plan a concrete slab for a shed foundation at 4 inches thick, with a thicker edge and a gravel base. For a different shed size, use the Concrete Slab Calculator as the tool for custom sizes.
Recommended shed foundation setup
Choose a 4-inch shed slab on a compacted gravel base, with a thicker edge around the perimeter to support the shed walls. Keep the slab a little larger than the shed so water drips clear of the walls, and slope it slightly away from any nearby building.
How much concrete for a shed slab at 4 inches thick
The table shows the main slab volume only: volume in cubic yards = length (ft) × width (ft) × 1/3 ÷ 27. The “order” column adds a 10% allowance. A thicker edge adds extra concrete, as shown in the worked example.
| Shed slab size | Area | Base volume | Order with 10% allowance |
|---|---|---|---|
| 8 × 10 ft | 80 sq ft | 0.99 cubic yards | 1.09 cubic yards |
| 10 × 12 ft | 120 sq ft | 1.48 cubic yards | 1.63 cubic yards |
| 12 × 16 ft | 192 sq ft | 2.37 cubic yards | 2.61 cubic yards |
Worked example: a 10 × 14 ft shed slab with a thicker edge
Main slab: area = 10 × 14 = 140 sq ft. Volume = 140 × 1/3 = 46.67 cubic feet = 1.73 cubic yards. Thicker edge: perimeter = 48 ft, with an extra 4 inches of depth over a 1-foot-wide edge, giving 48 × 1 × 1/3 = 16.00 cubic feet = 0.59 cubic yards. Subtotal = 1.73 + 0.59 = 2.32 cubic yards. With a 10% allowance: 2.32 × 1.10 = 2.55 cubic yards. Order about 2.55 cubic yards for this shed slab.
Method note
Measure the slab, not the shed, because the slab is usually a little larger. Add the thicker edge separately, as in the example, and keep the gravel base volume separate from the concrete volume.
Shed foundation questions
Why a thicker edge for a shed?
The edge carries the shed walls and resists edge cracking. The middle of the slab carries mainly floor loads, so it can stay at 4 inches.
Do I need a gravel base under a shed slab?
Yes. A compacted gravel base gives drainage and a level surface for the slab, and it helps stop the slab moving on soft ground.
Should a shed slab be larger than the shed?
A small overhang helps water drip clear of the shed walls. Too large an apron, however, can catch water against the shed, so keep the overhang modest and sloped.
Do I need reinforcement for a shed slab?
Wire mesh is a common choice for a shed slab. For a heavy shed or a slab carrying vehicles, check local guidance before choosing reinforcement.
For custom shed sizes, use the Concrete Slab Calculator.