The plan
The calculator sizes both layers and both ways of getting the concrete: a ready-mix wagon, or bags mixed on site. The bags come straight from the concrete calculator's mix table for a 1:2:4 mix, so the two pages cannot disagree.
It also puts steel mesh and the perimeter joint filler into the take-off, because a slab that size wants both — mesh for the slab itself, and a compressible board where it meets anything it is not bonded to.
Doing it, in order
- Mark the base out and check the diagonals are equal, then allow a little on each side so the base stands proud of the shed — it is what sheds water away from the frame.
- Excavate to the depth of the build-up and compact the bottom of the excavation.
- Lay and compact the MOT Type 1 in two layers to a finished 100 mm, checking the level as you go.
- Set the mesh on spacers so it sits in the middle of the slab, lapping the sheets and tying the laps with wire.
- Stand the expansion board around the perimeter, against anything the slab is not bonded to.
- Pour or mix the slab to 100 mm, tamp it level to a slight fall, and cover it while it cures.
Don’t forget
- Expansion board at the perimeter The calculator sizes it from the slab perimeter, 10.0 m here, and it goes against walls, kerbs and any existing slab.
- A fall, and somewhere for the water to go A base that is level, or falls towards the shed, sends water under the frame. That is a drainage judgement on your own ground, not a number this page can give you.
- The ground under the excavation Soft clay or made ground is not the same job as firm subsoil. No calculator can size a foundation without knowing what is underneath it.
- Curing time Concrete gains strength over days, not hours, and this is a job that cannot be rushed. Follow the mix supplier's own curing guidance rather than a figure from here.
Material take-off for this plan
Worked out by the site’s own Shed base for this plan’s dimensions, not typed in by hand.
The working
| Working | Value |
|---|---|
| Base area | 6.00 m² 3.00 m × 2.00 m — the shed footprint, with no overhang. A base that runs proud of the shed sheds water away from the frame. |
| MOT Type 1 sub-base | 1.19 t 6.00 m² × 100 mm × 1.80 t/m³ compacted, plus 10%. That 1.8 t/m³ is the same figure the gravel and paving calculators quote. ≈ 2 × 850 kg bulk bags. |
| Concrete slab | 0.600 m³ 3.00 m × 2.00 m × 100 mm, mix 1:2:4 — from the concrete calculator’s own mix table. |
| Ready-mix to order | 0.63 m³ ≈ 1.51 tonnes. Includes the 5% over-order the concrete calculator adds — short loads are costly. |
| Or mix on site | 8 cement + 17 sand + 32 agg 25 kg bags, straight from the concrete calculator’s figures for this mix. |
What to order
| Material | Quantity | Notes |
|---|---|---|
| MOT Type 1 sub-base | 1.19 tonnes | ≈ 2 × 850 kg bulk bags at 100 mm compacted. Weighted at 1.8 t/m³ — the figure the gravel and paving calculators use. |
| Ready-mix concrete | 0.63 m³ | One line if you are ordering a wagon. The mix comes from the concrete calculator’s own table. |
| Cement (25 kg bags) | 8 bags | For mixing on site — the concrete calculator’s own bag figure. |
| Sharp sand (25 kg bags) | 17 bags | ≈ 422 kg. |
| Ballast / 20 mm aggregate (25 kg bags) | 32 bags | ≈ 792 kg, or 1 bulk bags. |
| Steel reinforcement mesh A142 (4.8 × 2.4 m sheet) | 1 sheet | Rule of thumb: a 4.8 × 2.4 m sheet covers about 11.5 m², and the sheets lap one full square, so 1.12× the area is ordered. A shed base is often laid as a plain slab with no mesh at all. Put mesh in only where the slab is designed to carry it, and take advice if the shed is heavy or the ground is soft. |
| Mesh spacers / chairs | 12 spacers | Rule of thumb: 2 per m² to hold the mesh in the middle of the slab. Mesh lying on the ground or on the membrane reinforces nothing — it has to sit in the concrete. |
| Tying wire (1.6 mm coil) | 1 kg | Rule of thumb: about 1 kg per 20 m² of mesh, to tie the laps. Soft annealed tying wire, not a heavier gauge. |
| Expansion / perimeter joint filler board | 10 m | Rule of thumb: the slab perimeter, 10.0 m here, standing against walls, kerbs, a manhole or an existing slab. A slab pinned all the way round cracks somewhere else instead. |
A shed base is not a structural design, and this page does not check the ground, the drainage or the load. A small shed on firm, level, well-drained ground is one thing; a large shed, a workshop, a log store, a hot tub or anything heavy needs a base designed for the load and the ground it sits on — take advice from a competent person. 100 mm of concrete is a typical light-shed slab; a heavier load or softer ground needs more, and the concrete should be laid over a compacted sub-base, not straight onto soil.
A shed base, planned out FAQs
How much concrete do I need for a 3 m by 2 m shed base?
0.600 m³ of wet volume at 100 mm thick, from the shed-base calculator. Ordered as ready-mix that becomes 0.63 m³, because the calculator adds the 5% over-order the concrete calculator uses — short loads are expensive.
Do I need a sub-base under a shed base?
This plan uses 100 mm of compacted MOT Type 1, which comes to 1.19 tonnes for a 6 m² base with a 10% allowance — about two 850 kg bulk bags. It is what turns soft ground into something the slab can sit on.
How many bags of cement, sand and aggregate is that?
For a 1:2:4 mix the calculator returns 8 bags of cement, 17 of sharp sand and 32 of 20 mm aggregate, all 25 kg bags. That is the same mix table the concrete calculator uses.
Do I need reinforcement mesh in a shed base?
This plan includes one A142 sheet on spacers, plus tying wire for the laps. A 4.8 × 2.4 m sheet covers about 11.5 m² and the sheets lap one square, so serving a 6 m² base leaves an offcut.