What Concrete Mix Ratio Do I Need?

Common Mix Ratios

• C15 (Gen 1) — Strip foundations: 1:2:5 or 1:6 all-in ballast
• C20 (Gen 3) — General purpose: 1:2:4 or 1:5 all-in ballast
• C25 (ST2) — Driveways, garage floors: 1:1.5:3 or 1:4 all-in ballast
• C30 (ST3) — Structural, exposed: 1:1:2.5
• C35+ — Heavy duty: Specify by strength — use ready-mix

Materials per m³ (C20 / GEN 3 mix, 1:2:4)

Cement content below follows BS 8500-2 Table A.4 minimums: 240 kg/m³ for GEN 3 (internal, non-aggressive) and ~280 kg/m³ for RC20/25 or exposed work. Site-mixing 1:2:4 by volume typically under-cements against BS 8500 unless the cement quantity is checked explicitly by weight.

• Cement: 10 bags (25 kg) = 250 kg minimum for GEN 3; 11–13 bags (275–325 kg) for RC20/25 or exposed work
• Sharp sand: ~0.5 m³ (~800 kg)
• 20 mm aggregate: ~1.0 m³ (~1,600 kg)
• Water: ~150 litres (w/c ratio ~0.55–0.60; adjust for damp sand)
• Or all-in ballast: ~1.8 tonnes + 10–11 bags cement
• Important: if structural, order to BS 8500-1 designated strength (RC25/30, RC28/35, etc.) from a ready-mix plant — site-batched 1:2:4 is only reliable for paths, shed bases, and fence-post footings.

Tips for Mixing

• Always add water gradually — too much weakens the concrete
• The mix should hold its shape when squeezed, not be sloppy
• Use a cement mixer for quantities over 0.25m³
• Mix for at least 2–3 minutes after all ingredients are added

When to Use Ready-Mix

For quantities over 1m³, ready-mix concrete is more consistent, faster, and often more cost-effective. Specify the strength class (e.g., C20, C25) and any admixtures (e.g., retarder for hot weather, fibre reinforcement).

Last updated: April 2026. Ratios are by volume. For structural concrete, always follow an engineer’s specification.