Shed Base Calculator UK: Concrete, Paving Slab, Grid and Timber Bases
Choose a base type and your shed size, and this shed base calculator produces a complete materials list. It covers concrete slabs, paving slabs, plastic grids and timber frames. Every estimate also includes the excavation depth, the spoil volume, the typical installation price and the curing time.
More free UK building calculators
A shed base combines concrete, crushed stone and sometimes paving. These calculators cover each layer in more detail.
Cubic metres, bags, cement and ballast for slabs and footings.
MOT Type 1 tonnes, bulk bags, layers and dig-out depth.
Slabs, block paving, sub-base, bedding mortar and patio fall.
MOT Type 1, ballast, sand and gravel in tonnes and bulk bags.
Sharp, building and kiln-dried sand, plus sand and cement mixes.
Tonnage, coverage and driveway cost per m².
Bricks per m², mortar and cost for any wall.
Concrete, breeze and aircrete blocks, with mortar and cost.
Why shed bases go wrong
Picture this. The concrete has cured, the shed panels stand assembled, and the doors refuse to close. One corner of the base sits 15 mm lower than the others, because the hardcore underneath went in loose.
Most shed problems originate below the floor. An undersized base lets rainwater soak the bottom rail. A thin slab eventually cracks, and an uncompacted corner gradually subsides. Every one of those faults is considerably cheaper to prevent than to repair.
Plan the complete build-up: the calculator works out the hardcore, the slab and the excavation together. The finished base then sits level and slightly above the surrounding ground.
How much concrete for a shed base?
An 8 × 6 ft shed requires approximately 0.54 m³ of concrete, including 10% extra. That means 8 bags of cement and 1.17 tonnes of ballast.
How much does a concrete shed base cost?
A builder typically charges £400 to £690 for an 8 × 6 ft concrete base, plus VAT where applicable. Plastic grid bases cost approximately half as much.
How to use the shed base calculator
Select the tab for your preferred base, then choose a standard shed size or enter custom dimensions. Each estimate takes under a minute.
- Choose the base type. Concrete slab, paving slabs, plastic grid or timber frame. Each tab displays its own specific options.
- Select your shed size. Common UK sizes range from 4 × 3 ft to 20 × 10 ft. Switch to metres for customised dimensions.
- Check the base size. Concrete and paving bases default to the shed plus 100 mm. Grid and timber bases normally match the shed floor exactly.
- Specify the build-up. Choose the slab thickness and hardcore depth, or the paving size and bedding material.
- Add merchant prices if available. The calculator then compares mixing your own concrete, 20 kg bags and a ready-mix lorry.
It works as a concrete shed base calculator, a paving estimator and a complete materials list together.
Which shed base is best?
Every base must be firm, level and properly drained. The ideal choice depends on the building’s size and weight, the ground conditions, and your budget.
| Base | Best for | Typical laid price | Ready for the shed |
|---|---|---|---|
| Concrete slab | Workshops, log cabins, metal sheds and anything heavy | £60–£140 per m² | After about 7 days |
| Paving slabs | Small and medium sheds on level ground | £60–£100 per m² | Immediately on a sand bed |
| Plastic grid | Timber sheds and greenhouses up to about 10 × 8 ft | £30–£55 per m² | The same day |
| Timber frame (wooden base) | Sloping gardens and sheds that need ventilation underneath | £35–£60 per m² | The same day |
Typical 2026 UK prices from published trade guides. Smaller bases usually attract a minimum charge.
Our honest view? A grid or paving base usually suits a timber shed up to about 10 × 8 ft. It also costs considerably less. Choose concrete for workshops, log cabins, metal sheds and unstable ground.
What size should a shed base be?
The Heidelberg Materials shed base guide recommends adding 100 mm to the shed’s length and width. That leaves a small, consistent margin around the floor. Some builders prefer a base exactly the size of the floor, so rainwater drips straight off the cladding.
| Shed (nominal) | Shed in metres | Concrete base (+100 mm) | Base area |
|---|---|---|---|
| 6 × 4 ft | 1.83 × 1.22 m | 1.93 × 1.32 m | 2.5 m² |
| 7 × 5 ft | 2.13 × 1.52 m | 2.23 × 1.62 m | 3.6 m² |
| 8 × 6 ft | 2.44 × 1.83 m | 2.54 × 1.93 m | 4.9 m² |
| 10 × 8 ft | 3.05 × 2.44 m | 3.15 × 2.54 m | 8.0 m² |
| 12 × 8 ft | 3.66 × 2.44 m | 3.76 × 2.54 m | 9.5 m² |
| 16 × 10 ft | 4.88 × 3.05 m | 4.98 × 3.15 m | 15.7 m² |
Nominal shed sizes are often slightly smaller in reality, so measure the floor frame if you can.
Never let the base extend significantly beyond the walls. A wide ledge collects rainwater against the bottom rail and eventually rots it. Leave at least 500 mm behind the shed for maintenance and preservative treatment.
How thick should a shed base be?
Heidelberg Materials suggests at least 75 mm of concrete on 75 to 100 mm of compacted hardcore. Most builders specify 100 mm, and heavier buildings require considerably more.
| Building | Concrete | Hardcore | Dig depth |
|---|---|---|---|
| Small shed, firm ground | 75 mm | 75 mm | 150 mm |
| Most garden sheds | 100 mm | 100 mm | 200 mm |
| Shed on soft or clay subsoil | 100 mm | 150 mm | 250 mm |
| Log cabin or workshop | 150 mm with A142 mesh | 100–150 mm | 250–300 mm |
Dig depths include 25 mm of sand blinding, with the slab finishing 25 mm above the ground.
Our sand calculator converts the blinding sand into bags. Compact the hardcore in layers with a plate compactor. Then add sand blinding and a 1200 gauge damp-proof membrane, turned up inside the formwork. Our sub-base calculator explains the compaction layers and material density in more detail.
What concrete mix for a shed base?
The usual shed base concrete mix ratio is 1:5, cement to all-in ballast. It is roughly equivalent to C20 concrete. Dry materials shrink when mixed, so one cubic metre of concrete requires about 1.54 m³ of cement and ballast.
| Shed size | Concrete (+10%) | Cement and ballast (1:5) | Or 20 kg bags | MOT Type 1 |
|---|---|---|---|---|
| 6 × 4 ft | 0.28 m³ | 5 bags + 0.61 t | 28 | 0.56 t |
| 7 × 5 ft | 0.40 m³ | 6 bags + 0.87 t | 40 | 0.80 t |
| 8 × 6 ft | 0.54 m³ | 8 bags + 1.17 t | 54 | 1.08 t |
| 10 × 8 ft | 0.88 m³ | 13 bags + 1.92 t | 88 | 1.76 t |
| 12 × 8 ft | 1.05 m³ | 16 bags + 2.29 t | 105 | 2.10 t |
| 12 × 10 ft | 1.30 m³ | 20 bags + 2.84 t | 131 | 2.60 t |
100 mm slab on 100 mm of MOT Type 1, base size = shed plus 100 mm. Cement in 25 kg bags.
Mixing by hand suits bases under approximately half a cubic metre. Above that, a ready-mix lorry saves an exhausting day at the mixer. However, small deliveries often carry a part-load charge. Our concrete calculator compares bags, mixing your own and ready-mix for any slab.
How much does a shed base cost?
Builders usually price shed bases by area, with a minimum charge for smaller jobs. The Trade2Base 2026 shed base guide provides the ranges below, and the calculator applies them automatically.
| Base | Per m² | 8 × 6 ft | 10 × 8 ft |
|---|---|---|---|
| Concrete slab | £60–£140 | £400–£690 | £480–£1,120 |
| Paving slabs | £60–£100 | £300–£600 | £480–£800 |
| Plastic grid | £30–£55 | £150–£350 | £220–£410 |
| Timber frame | £35–£60 | £200–£350 | £260–£450 |
Typical installed prices. Difficult access, spoil removal and demolishing an old base cost extra.
Doing it yourself, the materials for an 8 × 6 ft concrete base cost roughly £200 to £300. Equipment hire, skip hire and your own labour add to that, so compare the totals carefully before you start.
Shed base calculator examples
We calculated each base with its default settings, so you can compare your own figures.
Example 1: concrete base for an 8 × 6 ft shed
2.54 × 1.93 m base, 100 mm slab on 100 mm of hardcore
Example 2: paving slab base for a 10 × 8 ft shed
450 × 450 mm slabs, bedded on 25 mm of sharp sand over 75 mm of hardcore
Example 3: plastic grid base for a 6 × 4 ft shed
500 × 500 mm grids, matching the shed dimensions
Example 4: timber frame for a 12 × 8 ft shed
100 × 47 mm treated joists at 400 mm centres
Example 5: log cabin base, 16 × 10 ft
150 mm slab with A142 mesh on 100 mm of hardcore
Building a block or brick shed instead? Our block calculator estimates the walls once the base is complete.
How to install a concrete shed base
- Mark out and excavate. Peg out the base, then excavate about 200 mm deep and 75 mm wider all round.
- Compact the hardcore. Spread MOT Type 1 in layers and compact each layer thoroughly.
- Blind and line. Add 25 mm of sand, then the damp-proof membrane, taped at every joint and turned up the sides.
- Set the formwork. Fix 25 mm boards with pegs at approximately one metre centres, then check them with a spirit level.
- Pour and tamp. Overfill by about 10 mm, tamp it down, and cover the slab in hot weather. Never pour when frost is forecast.
You can walk on the slab after about 48 hours and remove the formwork after five days. Our concrete curing guide explains timings in cold weather. Building a path alongside the shed as well? Our paving calculator handles patios and paths.
Shed base mistakes and planning rules
- Loose hardcore. Uncompacted stone settles gradually, and the slab subsides with it.
- No membrane. Moisture rises through the concrete and eventually rots the shed floor.
- An oversized base. The exposed ledge holds rainwater against the bottom rail.
- Building too soon. Wait about seven days before loading a newly poured slab.
- Inappropriate timber. Frames near the ground require timber treated for ground contact.
Most garden sheds fall under permitted development. Within 2 m of a boundary, the building must remain under 2.5 m high. Check the Planning Portal outbuildings guide before you build. For the hardcore alone, our MOT Type 1 guide covers depth and compaction.
Shed base calculator FAQs
How the shed base calculator works
The calculator adds the base margin to your shed dimensions, then works out each layer separately. Concrete uses a 1:5 mix at 1.54 times the volume, as in our concrete calculator.
MOT Type 1 weighs 2.0 t/m³ plus 10% for compaction, and excavated spoil weighs 1.8 t/m³. Paving beds and grid fill follow our paving calculator and the Ecobase installation guide. Prices are typical 2026 UK ranges.
Remember those doors that would not close? Compacted hardcore, a level slab and the correct base dimensions keep a shed square for many years. Prices checked in October 2026. Treat results as estimates, and check your shed maker’s base guidance.
Which base did you choose for your shed, and how has it performed? Tell us through our contact page. Real projects help us improve the calculator.