Concrete and ballast calculator

The Concrete Society’s DIY volume-batching table (published 3 June 2025) gives three concrete mixes: foundations (footings, blinding, fence posts) at 1:2.5:3.5 or 1:6 all-in ballast; general use (garage, shed base, domestic floors) at 1:2:3 or 1:5 ballast; and paving (paths, drives, steps) at 1:1.5:2.5 or 1:4 ballast. There is also a 1:4 bedding mortar for paving slabs. This is guidance for site mixing, not a structural specification.

In that Concrete Society table, assuming 25 kg bags and 20 mm aggregate: 5 bags per m³ for foundations, 6 for general use, and 8 for paving or bedding mortar. This calculator multiplies those figures by your volume and waste allowance, then rounds bags up for ordering.

For general-purpose concrete, Hanson / Heidelberg Materials Multicem data sheet gives 1 part cement to 4 parts all-in ballast by volume. The Concrete Society’s paving mix is also 1:4 ballast; its general-use ballast mix is 1:5. Use the mix that matches the job, and treat the quantities as estimates.

The quantities in the Society’s table do not include wastage. Tarmac has recommended adding about 10% extra cement when purchasing, and rounding aggregates up. The default 10% here is a starting point you can change, not a requirement.