Shed Base Calculator – What Do I Need for a Shed Base?
Pick your shed size and base type and we'll work out the sub-base, concrete, slabs, timber or grids — with a materials list and cost estimate.
Shed Base Calculator
Shed size
UK sheds are sold in feet but built to metric sizes — a nominal 8 × 6 is about 2.4m × 1.8m. Measure the actual base of your shed if you already have it.
Cross-section of this base
You need
0.475m³ concrete
2.4m × 1.8m base — 4.32 m²
- MOT Type 1
- 0.68 t
- 25kg pre-mix bags
- 44
- Concrete volume
- 0.475 m³
- DPM sheet
- 7.2 m²
How we worked this out
Your shopping list
1 × 850kg bulk bags of MOT Type 1
0.68 tonnes at 75mm compacted.
£55.00
1 × 1200 gauge DPM sheet (4 × 3m roll)
Needs about 7.2 m² with 300mm turn-up all round.
£35.00
44 × 25kg bags of pre-mixed concrete
0.011 m³ per bag.
£242
9 × Metres of 100mm timber formwork
Perimeter shuttering, pegged and levelled before the pour.
£54.00
Concrete shed bases at a glance (100mm slab, 75mm sub-base)
| Shed size | Base size | Area | Concrete + 10% | MOT Type 1 |
|---|---|---|---|---|
| 6 × 4 ft | 1.8 × 1.2 m | 2.16 m² | 0.24 m³ | 0.34 t |
| 7 × 5 ft | 2.1 × 1.5 m | 3.15 m² | 0.35 m³ | 0.50 t |
| 8 × 6 ft | 2.4 × 1.8 m | 4.32 m² | 0.48 m³ | 0.68 t |
| 10 × 8 ft | 3.0 × 2.4 m | 7.20 m² | 0.79 m³ | 1.13 t |
| 12 × 8 ft | 3.6 × 2.4 m | 8.64 m² | 0.95 m³ | 1.36 t |
| 14 × 10 ft | 4.2 × 3.0 m | 12.60 m² | 1.39 m³ | 1.98 t |
How this calculator works
Every base starts with the footprint. UK sheds are still advertised in feet but built to metric sizes, so a nominal 8 × 6 shed is about 2.4m by 1.8m — 4.32m². You can add an overhang if you want the base slightly larger than the shed.
For a concrete base we work out 75mm of compacted MOT Type 1 (area × depth × 2.1 tonnes per m³), a DPM sheet 300mm bigger all round, and a 100mm concrete slab with 10% waste. Below about 0.5m³ we suggest 25kg bags of pre-mix at 0.011m³ a bag; above that we switch to a 1:2:3 mix — 320kg of cement and 1.8 tonnes of ballast per cubic metre — or ready-mix.
For paving slabs it is the same sub-base, a 25mm sharp sand or mortar bed, and slabs with 5% waste. A timber frame gets treated bearers at 400mm centres on 40mm of gravel over membrane, and a plastic grid base gets grid panels, membrane and gravel infill.
Worked example: a concrete base for an 8 × 6 shed
2.4m × 1.8m gives 4.32m². At 100mm thick the slab is 4.32 × 0.1 = 0.432m³, or 0.475m³ once you add 10% waste. The 75mm sub-base is 4.32 × 0.075 × 2.1 = 0.68 tonnes of MOT Type 1, which is one 850kg bulk bag.
0.475m³ is right on the boundary. As bags that is about 44 × 25kg pre-mix bags — roughly £240 and a very long afternoon. Mixing your own with 152kg of cement (7 bags) and 0.86 tonnes of ballast is closer to £110, and a minimum ready-mix load will often be quoted around £180 delivered. For anything bigger than a 10 × 8, ready-mix wins every time.
Getting the base right
Dig out to firm subsoil — usually 150–200mm for a concrete base once you allow for sub-base and slab. Compact the MOT in two layers with a plate compactor rather than trying to whack 150mm in one go. Set your formwork to a string line with a very slight fall away from the door, then check the diagonals: if they match, the base is square and your shed will actually fit.
Lay the DPM before the concrete, turning it up the inside of the formwork. Pour, tamp with a straight edge, and float the surface — do not polish it glass-smooth or it will be lethal when wet. Cover with polythene and leave it at least three days before the shed goes on, a week in cold weather.
Tips from the trade counter
- Check the shed's actual base size. Manufacturers' nominal sizes vary by up to 75mm. Build the base to the real dimensions on the spec sheet.
- Keep timber off concrete. Bearers or a DPC strip under the shed floor let air circulate and stop the bottom rail rotting.
- Think about the door side. Leave a slab or path in front so you are not stepping into mud every time you fetch the mower.
- Plastic grids need a flat base. They are quick, but they do not level anything — the ground under them still has to be even and compacted.
- Order the sub-base slightly over. MOT compacts by about 20%, so a bag goes less far than the raw volume suggests.
UK rules for garden sheds
Most garden sheds are permitted development in England: single storey, eaves no higher than 2.5m, maximum overall height of 2.5m if it is within 2m of a boundary (4m dual-pitched otherwise), not forward of the principal elevation, and outbuildings covering no more than half the garden area.
Conservation areas, listed buildings, National Parks and some new-build estate covenants all have tighter rules, and Scotland, Wales and Northern Ireland differ. Always check with your local council before building.
Frequently asked questions
What is the best base for a garden shed?
A 100mm concrete slab on a compacted sub-base is the most durable and suits any shed. Paving slabs are quicker for small sheds, timber frames suit sloping ground, and plastic grids are the easiest DIY option for light sheds.
How thick should a concrete shed base be?
100mm of concrete over 75mm of compacted MOT Type 1 is right for a domestic shed. Go to 150mm of concrete if you are storing a ride-on mower, a motorbike or heavy machinery.
Should the base be bigger than the shed?
Opinions differ. A base the same size keeps rain dripping off the roof away from the timber. An overhang of 50–75mm makes the shed easier to position and stops the edges crumbling, but it can hold water against the walls if it is not laid to fall.
Do I need a damp proof membrane under a shed base?
Yes for concrete. A 1200 gauge DPM under the slab stops ground moisture rising into the concrete and then into your shed floor and contents.
How much concrete do I need for an 8x6 shed base?
An 8ft × 6ft shed is about 2.4m × 1.8m, which is 4.32m². At 100mm thick that is 0.43m³ of concrete, or 0.48m³ once you add 10% waste — around 44 bags of 25kg pre-mix, or a small ready-mix load.
Do I need planning permission for a shed?
Most sheds are permitted development if they are single storey, under 2.5m high when within 2m of a boundary, not forward of the house and do not cover more than half the garden. Different rules apply in conservation areas and to listed buildings — check with your local council.
Can I put a shed straight on grass or soil?
No. The floor will rot, the shed will sink unevenly and the doors will stop closing within a year. Even the lightest shed needs a level, free-draining base.