What is Grade of Concrete?
Grade of concrete denotes the strength required for construction. For example, the M30 grade indicates that the compressive strength required for construction is 30MPa. The very first letter in grade “M” is the mix and 30 is the required strength in MPa.
Based on various laboratory tests, the grade of concrete is presented in Mix Proportions. As an example, for the M15 grade, the mix proportion can be 1:2:4, where 1 is the ratio of cement, 2 is the ratio of sand, and 4 is the ratio of coarse aggregate based on quantity or weight of materials.
The strength is measured with concrete cubes or cylinders by civil engineers at the construction site. Cube or cylinders are made during casting of foundations, columns, beams, slabs, or any other structural member and after setting it is cured for a maximum of 28 days. Then compressive strength test is conducted to find the strength.
Normal grades of concrete are M15, M20, M25, and so on. For plain cement concrete works, usually, M15 is used. For reinforced concrete construction minimum M20 grade of concrete is used.
Concrete Grade

Mix Ratio  Compressive Strength MPa (N/mm^{2})  Compressive Strength Psi 
M5  `1 : 5 : 10  5 MPa  725 Psi 
M7.5  1 : 4 : 8  7.5 MPa  1087 Psi 
M10  1 : 3 : 6  10 MPa  1450 Psi 
M15  1 : 2 : 4  15 MPa  2175 Psi 
M20  1 : 1.5 : 3  20 MPa  2900 Psi 
M25  1 : 1 : 2  25 MPa  3625 Psi 
M30  Design Mix  30 MPa  4350 Psi 
M35  Design Mix  35 MPa  5075 Psi 
M40  Design Mix  40 MPa  5800 Psi 
M45  Design Mix  45 MPa  6525 Psi 
M50  Design Mix  50 MPa  7250 Psi 
M55  Design Mix  55 MPa  7975 Psi 
M60  Design Mix  60 MPa  8700 Psi 
M65  Design Mix  65 MPa  9425 Psi 
M70  Design Mix  70 MPa  10150 Psi 
There are two types of concrete mixes, nominal mix, and design mix. The nominal mix uses for normal construction works such as small residential buildings. One of the most popular nominal mix is the proportion of 1:2:4.
Design mixed concrete is those for which mix proportions finalize based on cylinder or cube lab tests for its compressive strength. This procedure is also called mix design. These tests conduct to choose a suitable mix based on locally available material to get the required strength as per structural design.