Concrete Mix Calculator
Calculate exactly how much cement, sand and coarse aggregate you need for site-mixed concrete. Works for any mix ratio — 1:2:4, 1:1.5:3, 1:2:3 and custom ratios. Metric and imperial.
🔨 Concrete Mix Calculator
📐 How It Works
Site-mixed concrete is calculated from the mix ratio (cement:sand:aggregate by volume). The dry ingredients volume is approximately 1.54 times the wet concrete volume (to account for voids being filled). Each material's volume is multiplied by its bulk density to get weight.
📋 Worked Example
1 m³ of 1:2:4 mix: Dry volume = 1.54 m³. Total parts = 7. Cement = (1/7)×1.54×1440 = 316kg = 13 × 25kg bags. Sand = (2/7)×1.54×1600 = 704kg. Aggregate = (4/7)×1.54×1550 = 1,363kg.
❓ Frequently Asked Questions
1:2:4 is a general-purpose mix suitable for most residential applications — paths, garden slabs, fence posts. It typically achieves N20 (20 MPa) strength. 1:2:3 is a stronger mix achieving approximately N25 (25 MPa) — used for structural slabs, footings and driveways. 1:1.5:3 achieves N32+ and is used for structural elements.
Ready-mix is better for pours over 0.5 m³ — more consistent quality, exact mix design, no labour for batching. Site-mixed suits remote locations, small pours, or where ready-mix is unavailable. Site mixing is labour-intensive and quality control is harder. For structural applications always use ready-mix from a certified batch plant.
Water-cement ratio of 0.45–0.55 is standard. For 350kg of cement: 160–190 litres of water. Never add more water on site to make it workable — it reduces strength significantly. Use a plasticiser (superplasticiser) if you need better workability without extra water.
When you mix dry cement, sand and aggregate, there are air voids between particles. When wet concrete is mixed, these voids are largely filled, reducing volume. The factor 1.54 accounts for this: 1.54 m³ of dry materials produces approximately 1 m³ of wet concrete.