Concrete Calculator — Cement, Sand & Aggregate Needed

Volume (m³) = Length × Width × Thickness. For a 5m × 4m slab at 150mm (0.15m) thickness: Volume = 5 × 4 × 0.15 = 3 m³. This wet volume is then converted to dry volume (multiply by 1.54, accounting for voids between aggregate particles that get filled during mixing) before calculating material quantities.

M20 concrete uses a nominal mix ratio of 1:1.5:3 (cement:sand:aggregate) by volume, giving a characteristic compressive strength of 20 N/mm² at 28 days. M15 uses 1:2:4, M25 uses 1:1:2. The "M" denotes the mix, and the number is the target strength in N/mm². Higher grades need proportionally more cement per unit volume.

For M20 (1:1.5:3 ratio): approximately 8 bags (50kg each) of cement per m³ of concrete. This is calculated as: dry volume × cement ratio ÷ sum of ratio parts ÷ volume of one cement bag (0.035 m³ per 50kg bag). Exact figures vary slightly based on aggregate size and water-cement ratio.