Anyone pouring a foundation slab, a path or a retaining wall needs to know the correct mixing ratio. Ready-mix concrete delivered by truck is ideal for large projects, but for smaller jobs mixing your own is worthwhile – if you know the right proportions. Our calculator works out the required amounts of cement, sand, gravel and water for your target volume.
Step by Step: How to Use the Concrete Mix Calculator
- Enter the concrete volume: How many m³ or litres of fresh concrete do you need?
- Choose the concrete class: C12/15 (light foundations), C20/25 (standard concrete), C25/30 (higher load requirements).
- Mixing ratio: C20/25 = 1 part cement : 1.5 parts sand (0–4mm) : 3 parts gravel (4–16mm).
- Water-cement ratio (w/c): Optimal 0.45–0.55. Less water = stronger concrete, but harder to work with.
- Read the quantities: Kilograms of cement, sand and gravel, plus litres of water for the desired volume.
Practical Examples
Garden paving foundation, 1m³ C20/25: Cement: 300 kg (6 bags of 50 kg), sand: 600 kg, gravel: 1,100 kg, water: 165 L. Fresh concrete weight: approx. 2,350 kg/m³.
Setting 3 fence posts, 20L concrete each: Total 60L = 0.06m³. Cement: 18 kg, sand: 36 kg, gravel: 66 kg, water: 10L.
Concrete mixer with 130L drum: Uses approx. 60% capacity = 78L fresh concrete per batch. For 1m³ you need about 13 batches.
Concrete Mixing Ratios
- C12/15: 1 cement : 2 sand : 4 gravel, w/c = 0.60
- C20/25: 1 cement : 1.5 sand : 3 gravel, w/c = 0.55
- C25/30: 1 cement : 1.2 sand : 2.5 gravel, w/c = 0.50
1 m³ fresh concrete = approx. 2,350 kg total weight
Frequently Asked Questions (FAQ)
Which concrete class do I need for which job?
C12/15: lean concrete, blinding layer. C20/25: standard for foundation slabs, retaining walls and garden paths. C25/30: basement floors, garages and steps with vehicle loads. For structural elements, always consult a structural engineer.
How long does concrete take to cure?
Concrete gains strength over weeks: day 1 approx. 30% of final strength; day 7 approx. 70%; day 28 approx. 100% (rated strength). Protect concrete from frost for the first 3 days and keep it moist.
Can I mix concrete without gravel?
Concrete without gravel is called mortar or screed. Mortar (cement + sand) has lower compressive strength – suitable for bricklaying, pointing and render, but not as structural concrete.
