How much concrete for a shed base?

Planning a shed base? Use our concrete calculator to work out exactly how much you need. This guide explains the key decisions.

What depth should I use?

Depth depends on the shed, the ground, and the manufacturer's instructions. Enter the depth you have planned — do not guess a structural thickness from this site. For structural work, check building regulations or a qualified professional.

Example: 3m × 2m × 0.1m slab

• Net volume: 3 × 2 × 0.1 = 0.60 m³
• With 10% waste (top of the Heidelberg 5–10% range): 0.66 m³ to order
• Blue Circle Multi-Purpose 20kg at 0.01 m³ per bag: 66 bags

Yield from the Tarmac Blue Circle technical sheet (one 20kg bag typically produces around 0.01 m³). Use the calculator for your own length, width and depth.

Ready-mix or bags?

Bags are easier for small DIY projects (under 1 m³). You can work at your own pace and store unused bags.

Ready-mix delivery is faster for larger bases but needs preparation: the truck arrives on schedule, you pour and level quickly, and there's often a minimum order. Check your supplier's order increments.

What about a sub-base?

Many bases sit on compacted hardcore so water can drain and the slab does not settle. That layer is separate from the concrete volume — do not include it in the calculator. Follow the shed instructions for whether you need one and how deep to go.

Why add waste allowance?

Ground may not be perfectly level, you'll have spillages, and it's better to have a bit left over than run short mid-pour. Heidelberg Materials UK recommends 5–10% extra for these factors.

Sources: Tarmac Blue Circle Multi-Purpose technical sheet (0.01 m³ per 20kg bag); Heidelberg Materials UK ready-mix volume FAQ (5–10% waste). Read 26 October 2026. Estimates only — check the manufacturer and, for structural work, building regulations or a qualified professional.