Shed Base Calculator — Materials for Concrete, Slabs or Gravel

A shed base should extend 50-100mm beyond the shed on each side. This gives a small overhang that prevents water dripping directly off the roof onto the base edge, which can cause erosion and undermine the base. For a 6×4 foot shed (1.83×1.22m), make the base roughly 1.98×1.37m with a 75mm overhang all round.

A concrete slab is the strongest and most durable option, ideal for larger sheds and workshops. Paving slabs on a sand bed are easier for DIYers and work well for smaller sheds. A gravel pad with timber frame is the quickest and cheapest option, and works well for standard garden sheds. All options need a compacted Type 1 sub-base for stability.

A concrete shed base should be at least 75-100mm thick for a standard garden shed. For larger workshops or sheds storing heavy equipment, go for 150mm. Always lay concrete on a compacted 100mm Type 1 sub-base. Use our Concrete Volume Calculator for detailed concrete quantities.

In most cases you don't need planning permission for a shed base in your garden, as sheds fall under permitted development rights. However, if your shed is in a conservation area, covers more than 50% of your garden, or exceeds certain height limits, you may need permission. Check with your local council planning department if unsure.

The number depends on your shed size and slab size. For 600×600mm slabs, divide your base area by 0.36 m² per slab. For a 6×4 foot shed (about 2.0×1.4m base with overhang), you need approximately 8 slabs before wastage, or 9 once this calculator's standard 10% wastage allowance is included. For more paving options, see our Paving Slab Calculator.

Figures below use this calculator's own defaults: a 75mm overhang each side, 100mm concrete depth, 50mm gravel depth, 10% wastage and 600x600mm paving slabs.

• 6x4 foot shed (1.83m x 1.22m): base area 2.71 m², needing 0.30 m³ of concrete (25 premix bags), 0.22 tonnes of gravel, or 9 paving slabs
• 8x6 foot shed (2.44m x 1.83m): base area 5.12 m², needing 0.56 m³ of concrete (47 bags), 0.42 tonnes of gravel, or 16 slabs
• 8x8 foot shed (2.44m x 2.44m): base area 6.70 m², needing 0.74 m³ of concrete (62 bags), 0.55 tonnes of gravel, or 21 slabs
• 10x8 foot shed (3.05m x 2.44m): base area 8.28 m², needing 0.91 m³ of concrete (76 bags), 0.68 tonnes of gravel, or 26 slabs
• 10x12 foot shed (3.05m x 3.66m): base area 12.18 m², needing 1.34 m³ of concrete (112 bags), 1.00 tonnes of gravel, or 38 slabs

Figures use precise foot-to-metre conversions, so re-typing the rounded metric sizes shown may shift a count by one bag or slab. The quick 8-slab estimate in the earlier FAQ uses approximate dimensions without the 10% wastage this table includes, which is why the two differ. Enter your shed's exact dimensions above for a precise figure. For a bare-footprint concrete example (no overhang or wastage, so its volumes are lower by design), see How Much Concrete for a Shed Base?

A compacted Type 1 MOT sub-base of at least 100mm is the standard minimum under a shed base, the same depth recommended for pedestrian paths and patios. Type 1 must be laid and compacted in layers no thicker than 100mm using a vibrating plate compactor, and conforms to the Specification for Highway Works Clause 803. This calculator adds a 100mm Type 1 allowance automatically for the concrete slab and paving slab options, editable in Advanced Options. For the gravel pad option, add your own sub-base allowance separately, since a gravel pad sits on membrane over firm, compacted ground rather than a metered MOT layer here. Heavier workshop bases or soft, clay-rich ground may need more, similar to a driveway's 150mm minimum. Use our Type 1 Sub-Base Calculator for a dedicated tonnage and bulk-bag breakdown.

For a gravel shed pad, lay gravel at least 50mm deep inside a timber frame, over a weed-suppressant membrane and firm, level ground. This calculator multiplies your base area (shed footprint plus the recommended overhang) by that depth, then converts to weight using a loose decorative-gravel density of 1.5 tonnes per m³ (the same figure our Gravel Calculator uses), though density varies roughly 1.4-1.8 t/m³ by stone type, so check your supplier's figure and scale the result if your stone is heavier or lighter. As an example, an 8x6 foot shed base (about 5.12 m² with the standard 75mm overhang) needs roughly 0.28 m³ of gravel at 50mm depth, around 0.42 tonnes once you include the usual 10% wastage. For gravel driveways and paths rather than shed pads, see our Gravel Calculator.

A general-purpose C20 (GEN 3) mix is the standard choice for a shed base: 1 part cement to 2 parts sharp sand to 4 parts 20mm aggregate by volume, or roughly 1 part cement to 5 parts all-in ballast. This calculator's site-mixed option works to a slightly richer 1:2:3 ratio. Either is fine for a shed base, since it's a non-structural, unreinforced slab rather than a load-bearing element; if you're building a heavier workshop base, follow a structural engineer's specification instead of a volumetric ratio. See our What Concrete Mix Ratio guide for a quick answer, or the full Concrete Mix Ratios reference for quantities per m³ across all the common mixes.

This calculator works out materials quantities rather than a price, but as a guide: bagged, ready-to-use concrete (25kg) typically costs around £4-9 per bag from UK builders' merchants and DIY retailers, so a small 6x4 shed base (about 25 bags with this calculator's defaults) costs roughly £100-225 in bagged materials alone. Delivered ready-mix concrete costs roughly £95-145 per m³ outside London and the South East, but most shed bases fall well under a typical 0.5-1 m³ minimum order, so bagged or site-mixed concrete is usually the more practical and cost-effective route for a shed base. If you're paying a groundworker to supply and lay the base rather than doing it yourself, budget for labour on top of materials: UK trade price guides put a fully installed concrete shed base, materials and labour together, at roughly £500-1,500 depending on size, though this varies by region and shed size.

Yes, for lighter structures some sheds are built on point supports, individual concrete or paving slabs set at the corners and at intervals under the floor bearers, rather than a full continuous base. This calculator is built for full-coverage bases (a complete concrete slab, a complete area of paving slabs, or a gravel pad), so it won't size a point-foundation layout. If your shed comes with a manufacturer's base plan, follow its pier spacing and block count exactly. Guidance on spacing varies with shed size, floor construction and ground conditions, so there's no single figure that fits every shed; as a general rule, heavier sheds, workshops and anything with a poured or fitted floor are safer on a full base like the ones this calculator sizes, while point foundations suit lighter, factory-floored sheds only. For slab quantities if you go the paving-slab route, see our Paving Slab Calculator.