In order to calculate amount of cement, sand and aggregate required in 1m3 of concrete , you have to know about different grades of concrete. Based on strength Different grades of concrete is classified into M5, M7.5, M10, M15 etc., wheras M stands for Mix and the number behind M stands for characteristic Compressive strength(fck) of the concrete in 28 days when checked in direct compression test. Tthe ratio for M20 grade of concrete is 1:1.5:3 that mean 1 part of cement, 1.5 part of sand (fine aggregate) and 3 parts of aggregate (crushed stone) in volume and then batched for mixing. To know the Concrete Mix Design follow below:-
Mix Design (M) = Cement: Sand :Aggregate