The grade of concrete is nothing but a strength of concrete of that particular mix that is a mixture of cement, sand, aggregate, and water in 28days. M denotes the grade of concrete, and M stands for the mix.
For example, for M5 grade, the mix proportions 1:5:10 where 1 is the cement ratio, 5 is the ratio of sand, and 10 is the ratio of coarse aggregate. Based on the volume or weight of materials, at least three concrete cubes of 150 ×150 × 150 mm are cast for each mix at 27 degrees Celsius, and they are tested in a laboratory at 28 days.
Its mean strength is calculated at 28 days, and that is the strength of concrete for that particular concrete mix ratio.
Density of Cement, Sand and Aggregate