How Much Concrete for a Shed Base?
A shed base is a small slab, but it sits right at the awkward point between bags and a truck.
A 10×12 shed base at 4 inches needs 1.63 cubic yards of concrete — 44 cubic feet, or 74 bags of 80 lb. Above one cubic yard you are past the supplier's minimum anyway, so two yards of ready-mix is usually the sensible order.
Estimates only. Textured walls, high ceilings, sprayers and major colour changes can all increase the amount you need. Always buy slightly more than the calculation suggests.
Concrete needed by shed size
| Pour | Volume | Cubic yards | Bags | Cheaper |
|---|---|---|---|---|
| Small shed, 6 x 8 at 4" | 17.6 cu ft | 0.7 cu yd | 30 × 80 lb | Ready-mix |
| Garden shed, 8 x 10 at 4" | 29.3 cu ft | 1.1 cu yd | 49 × 80 lb | Ready-mix |
| Standard shed, 10 x 12 at 4" | 44 cu ft | 1.6 cu yd | 74 × 80 lb | Ready-mix |
| Standard shed, 10 x 12 at 5" | 55 cu ft | 2 cu yd | 92 × 80 lb | Ready-mix |
| Large shed, 12 x 16 at 4" | 70.4 cu ft | 2.6 cu yd | 118 × 80 lb | Ready-mix |
| Workshop, 16 x 20 at 5" | 146.7 cu ft | 5.4 cu yd | 245 × 80 lb | Ready-mix |
10 % waste. Sheds take a 4-inch slab over 4 inches of compacted gravel — the gravel is not optional.
Make the slab bigger than the shed
This is the one thing people get wrong with shed bases, and it is expensive to fix later.
Pour the slab at least 6 inches larger than the shed footprint on every side. The shed's floor frame and cladding overhang the base, and if the slab stops exactly at the shed edge, water runs off the roof, down the walls, and straight onto the slab edge — where it sits and works its way under.
A 10×12 shed therefore needs an 11×13 slab. That is 143 square feet instead of 120 — about 19 % more concrete, and worth every bag of it.
A shed needs a base, not just a slab
Concrete is the expensive way to do this, and it is not always the right one. Three options, in ascending order of cost and permanence:
- Timber bearers on compacted gravel. Cheapest and fastest. Fine for a small shed on well-draining ground. The floor sits above the ground and stays dry.
- Paving slabs on a sand and gravel base. Good for medium sheds, easily lifted and relaid, and much cheaper than concrete.
- A concrete slab. The right answer for a heavy shed, a workshop with machinery, or anywhere you want a solid floor. Also the only option if you need to anchor the shed against wind uplift.
If you do pour concrete, you still need 4 inches of compacted gravel underneath and a damp-proof membrane over the slab before the shed goes down. Concrete wicks moisture, and a timber floor laid straight onto a slab will rot from below.
Frequently asked questions
How much concrete for a 10x12 shed base?
About 44 cubic feet with waste, which is 1.63 cubic yards or 74 bags of 80 lb mix, at 4 inches thick.
How thick should a shed base be?
Four inches for a normal garden shed. Five inches if it is a workshop with heavy machinery or you will store anything very heavy.
Should the slab be bigger than the shed?
Yes. Make it at least 6 inches larger on every side so roof runoff lands on the ground and not on the slab edge. A 10x12 shed wants an 11x13 slab.
Do I need gravel under a shed base?
Yes — 4 inches of compacted gravel, plus a damp-proof membrane over the slab before the shed goes down, or the timber floor will rot from underneath.