M10 M15 M20 CONCRETE

M10, M15, M20 etc. are all grades of concrete, The 'M' denotes 'Mix' followed by a number representing the compressive strength of that mix in N/mm². A mix is a specific ratio in which cement, fine(sand) and coarse aggregate(gravel) are mixed.  

M15 means a 150 mm cube cast from the concrete mix designed for the number. Continuously, M10, M20 is 100mm & 200mm cube cast for the concrete. For example, M20 concrete is supposed to have a compressive strength of at least  20 N/mm², 28 days after casting.

Concrete Mix Design
Grade of Cement	Mix Ratio	Water per bag (50kg ) of Cement
M 5	1 : 5 : 10	60 litre
M 7.5	1 : 4 : 8	45 litre
M 10	1 : 3 : 6	34 litre
M 15	1 : 2 : 4	32 litre
M 20	1 : 1.5 : 3	30 litre
M 25	1 : 1 : 2
M 30	Design