how we calculate of Sand, cement and aggregate of M20 ratio or other ratio?

how we calculate of Sand, cement and aggregate of M20 ratio or other ratio?

M20 = 1:1.5:3
Volume = 1+1.5+3=5.5
Total volume ingredients for using =1.57
Volume of broken stone Require = (3/5.5) x 1.57 = 0.856 m3
Volume of sand Require = (1.5/5.5) x 1.57 = 0.471 m3
Volume of cement = (1/5.5) x 1.57 = 0.285 m3
= 0.285 x1440 = 411 kg

For 1m3 of M20 (1:1.5:3)
Broken stone = 0.856 m3
Sand = 0.472 m3
Cement = 8.22 bag   32.5