Concrete Mix Ratio Calculator (Cement, Sand, Aggregate)

Concrete Mix Guide (UK)

Choosing Mix Strength

UK concrete strength classes (cube strength at 28 days, BS EN 206 / BS 8500): C8/10: weak — non-structural blinding, mass fill. C12/15: paths, garden edging. Limited use. C16/20: general purpose paths, garden walls, drives. Most common DIY use. Typical 1:2:4 mix (cement:sand:aggregate). C20/25: house foundations, slabs, drives carrying vehicles. 1:1.5:3 mix. Domestic standard. C25/30: load-bearing structural elements. 1:1.5:2.5 mix. Most domestic structural work. C30/37: enhanced structural. Lin

Mix Ratios Explained

Volume-based mix ratios (cement:sand:aggregate). Common ratios: 1:3:6 — weak general fill. C8-C12. 1:2:4 — general purpose paths, garden walls. C20. 1:1.5:3 — domestic foundations, slabs. C25-C30. 1:1:2 — high strength. C35-C40. Why both ratios and strength classes: ratios are practical for site mixing. Strength classes are technical specifications used by engineers. They overlap — a 1:1.5:3 mix typically achieves C25/30 under good control. Weight-based mixing more accurate (volume varies with h

Water and Workability

Water-to-cement ratio is THE most important factor in concrete strength. Lower w/c ratio = higher strength but harder to work. Practical ratios: 0.45-0.55 — strong, but stiff and harder to place. 0.55-0.65 — practical site mix balance. Typical DIY foundations. 0.65-0.75 — very workable but lower strength. Only for non-structural work. Over-watering is THE most common DIY mistake. Concrete that flows easily has been heavily over-watered. Strength can drop by 30-50% with just an extra litre per ba

Site Mixing vs Ready Mix

Site mixing (mixer or by hand): for small jobs (under 2-3 m³). DIY foundations, garden walls, small slabs. £80-150 per m³ in materials. Mixer hire £20-40/day. Labour intensive — typically 2 people working hard. Quality control variable. Ready-mix delivery: typical UK minimum 1-2 m³. Up to 8 m³ per truck. £100-180 per m³ including delivery. Quality controlled at batching plant. Saves significant time and physical effort. Recommended for: slabs over 2 m³, all structural work, sloping or difficult-