How the calculation works
The wet volume of concrete (length × width × thickness) is multiplied by 1.54 to get the dry volume of materials. The dry volume is split by the mix ratio, for example 1:1.5:3 for M20. Cement is converted to 50 kg bags at 1,440 kg/m³.
For 1 m³ of M20 nominal mix this gives about 8 bags of cement, 15 cft of sand and 30 cft of aggregate.
IS 456 allows nominal mixes only up to M20. For M25 and above, use a design mix tested by a lab.