Concrete for a 12 × 8 ft shed base, by thickness
A 12 × 8 ft shed is a large workshop or a store for garden machinery and furniture. The base here is 3.6 × 2.4 m, converted at 0.3 m per foot, which leaves it marginally smaller than the shed floor as most shed makers ask (check your shed's instructions). The figures include 10% for waste, and the ready-mix column rounds up to the next quarter m³. The cement, sand and aggregate are for mixing it yourself at 1:2:4.
| Thickness | Concrete | Ready-mix to order | Cement (25 kg bags) | Sharp sand | 20 mm aggregate |
|---|---|---|---|---|---|
| 100 mm | 0.95 m³ | 1.00 m³ | 13 | 0.67 t | 1.25 t |
| 125 mm | 1.19 m³ | 1.25 m³ | 16 | 0.84 t | 1.57 t |
| 150 mm | 1.43 m³ | 1.50 m³ | 19 | 1.00 t | 1.88 t |
Which mix, and how much sand and cement
Most shed bases are a 1:2:4 mix by volume. A 1:1.5:3 mix is stronger and uses more cement. These are the materials for the whole 3.6 × 2.4 m base at 100 mm, with 10% waste.
| Mix (cement : sand : aggregate) | Cement (25 kg bags) | Sharp sand | 20 mm aggregate |
|---|---|---|---|
| 1:2:4 (general purpose) | 13 | 0.67 t | 1.25 t |
| 1:1.5:3 (stronger) | 16 | 0.64 t | 1.20 t |
Mix it yourself or order ready-mix?
At 0.95 m³ most people mix it themselves: 13 bags of cement with 0.67 tonnes of sharp sand and 1.25 tonnes of aggregate, ideally with a hired mixer and a helper so the whole base is placed in one go. Get a ready-mix or mix-on-site quote too, and compare.
Sub-base, membrane and shuttering
Dig out the soil, then lay and compact about 100 mm of MOT Type 1 (2.09 tonnes for this base, or 3.14 tonnes at 150 mm). An optional damp-proof membrane needs about 10 m² with overlaps, and the shuttering boards run 12.0 m around the edge. Check for buried services before you dig, using the free LSBUD service. The shed base materials guide has the full shopping list and the steps.
Want to change anything else? The calculator above is the full Shed Base Materials Calculator (UK): edit any number and the results update instantly.
Tools & materials for this project
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Frequently asked questions
How much concrete do I need for a 12x8 shed base?
A 12 × 8 ft shed base is about 3.6 × 2.4 m (8.64 m²). At 100 mm thick it is 0.86 m³ before waste and 0.95 m³ with 10% extra, so order 1.00 m³ of ready-mix. At 150 mm it is 1.43 m³ with waste.
How many bags of cement for a 12x8 shed base?
About 13 bags of 25 kg cement for a 1:2:4 mix at 100 mm thick with 10% waste, plus 0.67 tonnes of sharp sand and 1.25 tonnes of 20 mm aggregate. A stronger 1:1.5:3 mix needs 16 bags.
What concrete mix should I use for a 12x8 shed base?
A general-purpose 1:2:4 mix by volume (one part cement, two of sharp sand, four of 20 mm aggregate) is the usual choice for a garden shed base. For a heavy workshop, use 1:1.5:3. If you order ready-mix, tell the supplier it is for a shed base and let them recommend the mix.
How much MOT Type 1 for a 12x8 shed base?
About 2.09 tonnes for a 100 mm compacted sub-base, or 3.14 tonnes at 150 mm on soft ground.
Related sizes and guides
Related sizes
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.