Concrete Mix Ratio for various grades of concrete

What are the concrete mix ratio for various grades of concrete available i.e. M5, M10, M15, M20, M25, etc. Also can anyone tell me what do you mean by M20 grade of concrete? I think its the ratio of cement, sand, aggregate that differentiates the grade of concrete but I am still not sure. Can someone please explain it?

Answer: Concrete are classified as M5, M10 or M20 on the basis of strength where M stands for Mix and numerical value stands for characteristic compressive strength of the concrete in 28 days. However, standard mix ratio is given for each grade of concrete with which we can obtain desired strength of concrete in 28 days. The ratio of Cement, Sand, Aggregate for each grade is defined below:

GRADE	MIX RATIO
M5	1 : 5 : 10
M7.5	1 : 4 : 8
M10	1 : 3 : 6
M15	1 : 2 : 4
M20	2 : 3 : 6 (or 1:1.5:3)