Concrete Mix Ratio Calculator (Cement, Sand, Aggregate)
Calculate concrete materials (cement, sand, aggregate, water) for any volume and mix strength. Includes UK BS 8500 mix designations.
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Concrete Mix Guide (UK)
What should I know about 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. Lintels, foundations for extensions, and other load-bearing structural work typically specify this grade or higher.
What should I know about 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 is more accurate, since volume varies with how loosely or tightly aggregate is packed, but volume-based mixing (using a bucket or shovel ratio) is far more practical for most DIY jobs and accurate enough for non-structural work.
What do I need to know about 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 or two of water per batch, which is why resisting the temptation to add more water for easier mixing matters so much.
What's the difference between Site Mixing and 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 cubic metres, all structural work, and sloping or difficult-to-access sites where hand-mixing consistent batches would be impractical.