Concrete mix ratios explained
A mix ratio is simply cement : sand : coarse aggregate by volume. The classic general-purpose mix is 1:2:3, which gives roughly C25 strength — strong enough for shed bases, paths, footings and driveways.
Common UK mixes
1:3:6 (about C10) is a weak mix for kerb bedding and blinding. 1:2:4 (about C20) suits paths, shed bases and non-structural slabs. 1:2:3 (about C25) is the standard domestic mix for footings, driveways and garage bases. 1:1.5:3 (about C30) is used for heavy-duty slabs and anywhere with vehicles or heavy loads.
How much of each you need
One cubic metre of finished concrete typically takes about 320kg of cement (thirteen 25kg bags) plus roughly 1.8 tonnes of all-in ballast for a 1:2:3 mix. Always add 10% waste — concrete cannot be topped up an hour later.
Bagged pre-mix is easier for small jobs: a 25kg bag makes about 0.011m³, so a 0.5m³ base needs 45 bags. Above roughly 1m³, order ready-mix. It is cheaper, stronger and arrives in one go.
Water and workability
Use the least water that still lets you place and compact the mix — around 0.5 litres per kg of cement. Extra water makes concrete easier to pour and permanently weaker. Cover a fresh pour and keep it damp for three days in warm weather, and protect it from frost in winter.