How this calculator works
This runs our concrete and gravel calculators on the base. The concrete is the area times the thickness in metres, with your waste allowance, and the DIY mix splits the dry volume (1.54 times the finished volume) into cement, sharp sand and aggregate. The sub-base is MOT Type 1 at 2.2 tonnes per m³.
Worked example: a base for an 8 × 6 ft shed (2.4 × 1.8 m) at 100 mm thick needs 0.48 m³ of concrete with 10% extra (order 0.50 m³ of ready-mix). Mixed yourself at 1:2:4 that is 7 bags of 25 kg cement, 0.33 tonnes of sharp sand and 0.63 tonnes of aggregate, on 1.05 tonnes of MOT Type 1.
Assumptions and sources
- Check the shed maker's instructions for the base size: most want a base the same size as the floor, or marginally smaller. Cover the concrete and keep it damp for about a week, and wait a few days before you build the shed on it. Check for buried services before you dig, using the free LSBUD service.
The formula and a worked example are shown above, so you can check any result by hand. Read how we calculate and check our numbers. Results are estimates for planning: supplier products vary, so check the size printed on the label.
Shed bases by shed size (100 mm slab, 100 mm MOT Type 1, 1:2:4 mix)
With 10% waste.
| Shed | Concrete | Cement bags | Sharp sand | Aggregate | MOT Type 1 |
|---|---|---|---|---|---|
| 6 × 4 ft (1.8 × 1.2 m) | 0.24 m³ | 4 | 0.17 tonnes | 0.31 tonnes | 0.52 tonnes |
| 7 × 5 ft (2.1 × 1.5 m) | 0.35 m³ | 5 | 0.24 tonnes | 0.46 tonnes | 0.76 tonnes |
| 8 × 6 ft (2.4 × 1.8 m) | 0.48 m³ | 7 | 0.33 tonnes | 0.63 tonnes | 1.05 tonnes |
| 8 × 8 ft (2.4 × 2.4 m) | 0.63 m³ | 9 | 0.45 tonnes | 0.84 tonnes | 1.39 tonnes |
| 10 × 6 ft (3 × 1.8 m) | 0.59 m³ | 8 | 0.42 tonnes | 0.78 tonnes | 1.31 tonnes |
| 10 × 8 ft (3 × 2.4 m) | 0.79 m³ | 11 | 0.56 tonnes | 1.05 tonnes | 1.74 tonnes |
| 12 × 8 ft (3.6 × 2.4 m) | 0.95 m³ | 13 | 0.67 tonnes | 1.25 tonnes | 2.09 tonnes |
| 12 × 10 ft (3.6 × 3 m) | 1.19 m³ | 16 | 0.84 tonnes | 1.57 tonnes | 2.61 tonnes |
| 14 × 10 ft (4.2 × 3 m) | 1.39 m³ | 18 | 0.98 tonnes | 1.83 tonnes | 3.05 tonnes |
Tips before you buy
- Under about a cubic metre it is practical to mix by hand or with a mixer. Above that ready-mix is easier, but check the minimum load and the delivery charge.
- Do not skip the sub-base. A slab on soft ground will crack and settle.
- Order a little more than you calculate. Uneven ground eats concrete.
- Do not pour in frost or heavy rain, and do not let the surface dry out too quickly in hot weather.
- Most garden sheds are permitted development, but there are limits on height and on how much of a garden outbuildings can cover.
Quick answers by size
Tools & materials for this project
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Frequently asked questions
How much concrete do I need for an 8x6 ft shed base?
An 8 × 6 ft shed is about 2.4 × 1.8 m. A 100 mm base is 0.43 m³ before waste and 0.48 m³ with 10% extra, so order 0.50 m³ of ready-mix. For that size, many people mix it themselves.
How thick should a shed base be?
A 100 mm thick slab on a compacted sub-base is the usual choice for a garden shed. Go thicker for a large, heavy shed or a workshop, or on poor ground, and follow the shed maker's instructions.
How many bags of cement for a shed base?
For the 8 × 6 ft base mixed 1:2:4 you need 7 bags of 25 kg cement, 0.33 tonnes of sharp sand and 0.63 tonnes of 20 mm aggregate, with 10% waste.
What concrete mix is best for a shed base?
A general-purpose 1:2:4 mix by volume: one part cement, two parts sharp sand and four parts 20 mm aggregate. That is what this calculator uses. For a heavy workshop or poor ground use a stronger 1:1.5:3 mix, which for the 8 × 6 ft base takes 8 bags of cement instead of 7. If you order ready-mix, tell the supplier it is for a shed base and let them recommend the mix.
Should I use ready-mix concrete for a shed base?
For most garden sheds the base is under a cubic metre (0.48 m³ for 8 × 6 ft, 0.79 m³ for 10 × 8 ft), which is practical to mix yourself in a hired mixer. Ready-mix or a mix-on-site delivery makes sense for larger bases, but ask about minimum loads and short-load charges first.
How much sand and cement for a shed base?
For an 8 × 6 ft base 100 mm thick at 1:2:4: 7 bags of 25 kg cement, 0.33 tonnes of sharp sand and 0.63 tonnes of aggregate. For 10 × 8 ft: 11 bags, 0.56 tonnes of sand and 1.05 tonnes of aggregate. Both include 10% waste.
Do I need a sub-base under a concrete shed base?
It is good practice: about 100 mm of compacted MOT Type 1 stops the slab settling and lets water drain. For the 8 × 6 ft base that is about 1.05 tonnes.
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Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.