How to size a shed base
Start with the shed’s floor, not its roof. UK sheds are sold in feet, such as 8 × 6 or 10 × 8, and those sizes are nominal: the real floor can differ by a few centimetres, and the roof overhangs the walls. Check the floor size in your shed’s instructions, then pick the nearest standard size, or choose Another size and type in the floor length and width.
Wickes suggests a base the same size as the shed or slightly larger. Extra all round adds the same margin to every side, so 50 mm all round makes the base 100 mm longer and 100 mm wider than the floor. The calculator starts with no margin, a base the same size as the shed.
Lengths can be typed in metres or in feet and inches; switch at the top of the calculator. Depths are always in millimetres.
Recommended shed base depths
A shed base has two layers: a compacted MOT type 1 sub-base and, on top, either a concrete slab or paving slabs on a mortar bed. These depths come from the Wickes and B&Q shed base and concrete guides.
| Use | Top layer | Type 1 sub-base, compacted | Apply |
|---|---|---|---|
| Concrete slabCement and ballast, or ready-to-use bags. | 75–100 mm of concrete | 75 mm | |
| Paving slabsLaid like a small patio. | Slabs on a 30–60 mm mortar bed | 75 mm |
Wickes builds its shed base with concrete at the thinner end of that range over 75 mm of sub-base; B&Q’s concrete guide gives the thicker figure for a shed, and that is where the calculator starts. For a slab base it uses the same full mortar bed as a patio.
Treat these as a guide for an ordinary garden shed. Soft ground, slopes, drainage and heavy contents such as a workshop all change what a base needs, so check yours with your supplier or a professional before you dig.
Shed base size calculator: from feet to metres
Standard sizes are converted at 0.3048 m to the foot, so an 8 × 6 ft shed needs a base of 2.44 m × 1.83 m, a little under 4.5 m², before any margin. The result shows the base size in metres at the top, which is what you mark out on the ground with pegs and string.
Used as a shed base size calculator, the margin box does the rest: add the margin you want and the base size, the area and every material update together. If your shed maker gives base dimensions of its own, use those with Another size rather than the nominal floor size.
Concrete for a shed base
The concrete volume is the base area × the slab thickness, plus 10%, the allowance B&Q adds to concrete. Then there are two ways to buy it, and the Concrete choice in step 2 switches between them.
Mixing it yourself: cement and ballast
B&Q mixes shed base concrete at one part cement to 4 parts ballast. Its table shows that a cubic metre of concrete takes a full cubic metre of all-in ballast plus the cement: once water goes in, the cement fills the gaps between the stones. So the dry materials come to 1.25 times the finished volume. The calculator uses that factor, splits the dry volume at the mix ratio and weighs each part: ballast at a typical 1.5–1.8 t/m³, cement at a density worked out from B&Q’s concrete table.
For ballast on its own, for a path edge or a footing, use the aggregate calculator.
Ready-to-use bags of concrete
Bags of ready-mixed concrete only need water, which is handy for a small base. Each 20 kg bag makes only about 0.01–0.0125 m³ of concrete, according to Tarmac and Carlton, and the calculator uses the lower figure. Even a small shed base takes a lot of bags, so compare the two options before you decide.
Paving slab shed bases
A slab base is laid like a small patio: slabs on a full bed of sharp sand and cement over the type 1. The calculator offers 600 mm and 450 mm square slabs, counts them without joints and adds the allowance for cuts. If the base is not a whole number of slabs, the last row is cut.
The mortar bed is worked out like a patio bed, at 3–4 parts sand to one of cement. For a patio or path next to the shed, the patio calculator adds joints and jointing compound as well.
What this calculator leaves out
There is no damp-proof membrane in the list. We have not found a UK guide that gives a quantity for one under a shed base, so follow your shed maker’s instructions. Plastic grid bases are not covered either. The list also leaves out the timber for the formwork round a concrete base, and the pegs and string for setting out.
Worked example: a concrete base for a 10 × 8 shed
Here is the full calculation for a 10 × 8 ft shed on a concrete base mixed from cement and ballast, with no extra margin, the starting depths and the standard extra allowance. Every line comes from the same code that runs the calculator above.
- Shed 10 × 8 ft: 3.05 m × 2.44 m = 7.43 m²
- Base (3.05 m + 2 × 0 mm) × (2.44 m + 2 × 0 mm) = 3.05 m × 2.44 m = 7.43 m²
- Concrete 7.43 m² × 0.1 m + 10% = 0.818 m³
- Dry materials 0.818 m³ of concrete × 1.25 = 1.02 m³
- Cement (1 part in 5) 1.02 m³ ÷ 5 × 1.4 t/m³ = 286 kg → 12 25 kg bags
- All-in ballast (4 parts in 5) 1.02 m³ × 4 ÷ 5 × 1.7 t/m³ = 1.39 t
- Sub-base: MOT type 1 sub-base 7.43 m² × 0.075 m × 2.1 t/m³ + 10% = 1.29 t
On the list:
- 1225 kg bags of cement286 kg needed
- 2bulk bags of all-in ballast (approx. 850 kg each)1.39 t needed
- 2bulk bags of MOT type 1 sub-base (approx. 850 kg each)1.29 t needed
Notice that the ballast and the sub-base are both bulk-bag jobs, while the cement is counted in bags: the mix puts several times as much ballast as cement into the concrete. Switch to Ready-to-use bags to compare: the same volume of concrete takes far more bags.
UK cement, ballast and concrete sizes
Cement comes in 25 kg bags and ready-to-use concrete in 20 kg bags. All-in ballast, sharp sand and MOT type 1 come in bulk bags of around 850 kg or 800 kg, depending on the merchant, and ballast and sand in small bags too.
| Product | Size | Good for |
|---|---|---|
| Cement, 25 kg bag | 25 kg | General purpose cement. |
| Ready-to-use concrete, 20 kg | 20 kg | Just add water; for small bases. |
| Bulk bag, 850 kg | 850 kg | The most common merchant bulk bag. About 10 m² of gravel at 50 mm. |
| Bulk bag, 800 kg | 800 kg | Sold by several DIY chains and merchants; weight can vary by about 5% with moisture. |
| Small bag, 22.5 kg | 22.5 kg | A common DIY-store “major bag” size. |
| Small bag, 25 kg | 25 kg | Merchant “handy bag” or trade pack. |
| 600 × 600 mm slabs | 600 × 600 mm | Single-size paving. |
| 450 × 450 mm slabs | 450 × 450 mm | Single-size paving. |
The list counts bulk bags of 850 kg. If your merchant’s bags are smaller, a large base may need one more: divide the tonnes in the result by their bag size and round up.
Common mistakes with shed bases
- Sizing the base to the roof. The roof overhangs the walls. Size the base to the floor, plus any margin you want.
- Trusting the nominal size. An 8 × 6 shed is not always exactly 8 ft by 6 ft. Check the floor size in the instructions.
- Forgetting the sub-base. Both kinds of base sit on compacted type 1, and it is on the list whichever you pick.
- Underestimating ready-mixed bags. One bag makes only a small amount of concrete. Check the bag count before you choose them for a big base.
- Ordering ballast for the concrete volume only. Dry ballast and cement take up more room than the concrete they make. The calculator allows for this using B&Q’s figures.
- Forgetting the formwork. A concrete base needs timber boards round the edge while it sets, and they are not on the list.
What else you'll need
- Type 1 for a path to the shed: the type 1 calculator works out the sub-base on its own.
- Ballast or sharp sand on their own: use the aggregate calculator.
- A patio beside the shed: the patio calculator does slabs, bed and jointing.
- Paint or treatment for the shed once it is up: the fence paint calculator covers sheds too.
- Timber for the formwork, pegs and string to mark out, and a long spirit level to check the base.
Last checked 28 October 2026