How cement, sand and aggregate are calculated
Dry volume first, then split by the mix ratio. Dry cement, sand and aggregate contain air gaps, so the dry mix takes up about 1.54 times the volume of the finished wet concrete. The calculator multiplies your wet volume by 1.54, then splits it by the mix ratio — M15 is 1 part cement : 2 parts sand : 4 parts aggregate. One cubic metre of M15 needs 6.29 bags of 50 kg cement (buy 7), 0.44 cubic metres of sand and 0.88 cubic metres of aggregate.
Mix grades: M10 (1:3:6) for low-strength work, M15 (1:2:4) for general use, M20 (1:1.5:3) for structural work, and M25 (1:1:2) for high strength. A 50 kg bag of cement is 0.035 cubic metres (1.25 cubic feet).
Working out the full pour from slab size? Use the concrete calculator.