What Concrete Mix Ratio Do I Need?
Quick Answer
General purpose (C20): 1 cement : 2 sand : 4 aggregate
Or 1:5 all-in ballast. By volume, using a bucket or shovel as your measure. Suitable for paths, shed bases, and general groundworks.
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 Mix)
- Cement: ~6 bags (25kg) = 150kg
- Sharp sand: ~0.35m³ (~600kg)
- 20mm aggregate: ~0.7m³ (~1,200kg)
- Water: ~150 litres (adjust for damp sand)
- Or all-in ballast: ~1.8 tonnes + 6 bags cement
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.