Concrete Mix Calculator UK — Cement, Sand & Aggregate

The starting point is your concrete volume: length × width × depth. A 2.4m × 1.8m shed base at 100mm thick gives 2.4 × 1.8 × 0.1 = 0.432m³. Add 10% waste and you need 0.475m³ to order.

Concrete mix ratios describe dry materials by volume. A 1:2:4 mix means 1 part cement, 2 parts sharp sand, and 4 parts coarse aggregate. The total parts are 1+2+4 = 7. Cement is 1/7 of the total, sand is 2/7, aggregate is 4/7.

There is a catch: dry materials are bulkier than wet concrete. A cubic metre of finished concrete requires about 1.54 cubic metres of dry materials combined. The calculator applies this factor automatically, so the material quantities shown are correct for the wet volume you entered.

For a 0.475m³ shed base using a 1:2:4 mix, you need approximately 152kg of cement (about 7 bags of 25kg), 300kg of sharp sand, and 600kg of coarse aggregate. Alternatively, replace the sand and aggregate with 900kg of all-in ballast at a 1:4 cement-to-ballast ratio.

Five mix ratios cover almost every domestic and light commercial project in the UK. Each corresponds to a standard concrete grade defined by BS 8500.

C20 is the default for most domestic projects. It handles foot traffic, garden structures, and domestic loading without problems, and it is the easiest ratio to mix by hand. Use C25 wherever vehicles will park or drive regularly. C30 is rarely site-mixed — if your project needs C30 or above, a designed ready-mix from a local supplier is more reliable than hand-batching.

These quantities are for one cubic metre of finished concrete, including the 1.54 dry volume correction factor. Use them as a reference when ordering materials for larger projects where it is simpler to think in per-m³ terms.

For ballast (all-in aggregate), replace the sand and aggregate columns with a single material. For C20, use 1 part cement to 4 parts ballast. For C25, use 1:3. Ballast simplifies ordering and is often cheaper per tonne. One bulk tonne bag covers roughly 0.6m³ of finished C20 concrete. Check our ballast calculator for exact quantities.

Shed base and garden structures: Use C20 (1:2:4). At 100mm thick with a compacted hardcore sub-base beneath, a C20 shed base handles foot traffic and the static load of a timber shed without risk of cracking. The mix is forgiving to batch by hand and allows plenty of working time before it stiffens.

Patios and garden paths: C20 again, at 75-100mm thick. For decorative concrete or exposed aggregate finishes, a slightly richer 1:1.5:2.5 mix helps workability when trowelling the surface. Lay patio concrete over 75-100mm of compacted MOT Type 1 sub-base.

Driveways and garage floors: Step up to C25 (1:2:3). Vehicles impose dynamic loads through four contact patches, and freeze-thaw cycles attack the surface over winter. The extra cement in C25 tightens the concrete matrix and reduces permeability. For a standard single-width driveway at 100mm thick, budget roughly 15-16 bags of cement per cubic metre.

Strip and pad footings: C20 suits most domestic footings under walls and fences. Foundation concrete for extensions should follow your engineer's specification — typically C25 minimum where the load from upper storeys is significant. Use our footing calculator for volume and material quantities.

Fence posts: A 1:2:4 mix works well, or dry-pack Postcrete for speed. For a standard 100mm post in a 200mm hole at 600mm depth, you need less than 0.02m³ — one 25kg premixed bag per post is the practical choice.

Keep the water-to-cement ratio below 0.5. That means no more than 12.5 litres of water per 25kg bag of cement. Too much water is the single most common cause of weak concrete. The mix should be stiff enough to hold its shape when squeezed in a gloved hand — not liquid, not crumbly.

Mix dry ingredients first. Combine cement, sand, and aggregate in the mixer drum without water until the colour is consistent throughout. Then add water in two or three increments, checking workability after each addition. This prevents lumps of dry cement sticking to wet aggregate.

In summer, mix concrete early in the morning. Heat accelerates the cement reaction and reduces your working time. Cover the mixer drum and any mixed concrete with a damp hessian sack between loads. On hot days, use cold water and keep aggregate in the shade.

For projects over 1m³, consider a ready-mix delivery. Site mixing that volume takes a full day for two people. A mini-mix lorry delivers to your formwork in 15-20 minutes, and the volume is guaranteed consistent throughout. Use the mix calculator above to confirm your volume, then ring local suppliers for a quote before committing to bags.