Grade of concrete is defined by the strength and composition of the concrete. It is also defined as the minimum crushing strength of 150mm concrete cube after 28 days of curing. It is denoted by prefixing M to desired strength in MPa. For example, if there is a concrete mix with compressive strength of 15MPa, then its grade will be represented by M15.
Each grade of concrete is to convert into various mix proportion. Like mix proportion of M20 concrete will be 1:1.5:3 and that of M25 concrete will be 1:2:4. It is written as cement:sand:aggregate. This mix proportion of concrete grade is divided into two types:
 Ordinary Mix Proportion
 Designed Mix Proportion
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Ordinary Mix Proportion
The concrete mix proportion in which no preliminary tests are performed for designing the mix is called ordinary mix proportion. This ordinary mix proportion includes M5, M7.5, M10, M15, M20, and M25. Whose mix proportion is given below:
Grade of Concrete  Mix Ratio (Cement:Sand:Aggregate) 
M5  1:5:10 
M7.5  1:4:8 
M10  1:3:6 
M15  1:2:4 
M20  1:1.5:3 
Design Mix Proportion
Type of mix proportion for concrete in which preliminary tests are performed for designing the mix is called design mix proportion. This mix proportion is designed to obtain economical and specified properties of concrete. It includes M30, M35, M40, M50, M55, M60 & M80.
Mix proportions for above grade is find out by following Indian Standard IS102622009 specifications.
Classification of grades and their uses
On the basis of compressive strength of concrete, grades of concrete are mainly divided into following four types that are:
 Lean Concrete
 Ordinary grade of concrete
 Standard grade of concrete
 High Strength grade of concrete
Lean Concrete Grades:
When liquid proportion of concrete is much higher than that of cement part, then the result is in Lean concrete. Grades for lean concrete include M5 and M7.5.

M5 Grade of concrete:
This grade of concrete comprises compressive strength of 5MPa or 5 N/mm^2 with mix proportion of 1:5:10 where 1 represents cement, 5 represents sand and 10 represents aggregate.
Uses: This is used for low strength work, plain cement concrete (PCC) work such as leveling course, bedding for footing, and to prevent direct contact of foundation concrete with soil.

5 Grade of Concrete:
This grade of concrete comprises compressive strength of 7.5 MPa with mix proportion of 1:4:8 where 1 represents cement, 4 represents sand and 8 represents aggregate.
Uses: This type of concrete grade is also used for low strength plain cement concrete work, bedding of footing, and for PCC Floor where movement of load occurs.
Also Read: What is Prestressed Concrete? – Pre tension – Post Tension – Applications
Ordinary Grades of Concrete:
In ordinary grade of concrete, it includes M10, M15 and M20 whose uses, and mix proportion is defined below.

M10 Grade of Concrete:
Mix proportion for M10 grade of concrete comprises 1:3:6 which includes 1 cement, 3 sand and 6 aggregates.
Uses: This type of concrete also comprises lesser portion of cement as compared to other parts that’s why it is used for non structural work, bedding for footing, to prevent direct contact of foundation concrete with soil, Patio slabs and in pathways.

M15 Grade of Concrete:
It consists of mix proportion of 1:2:4 (Cement: Sand: Aggregate) and its compressive strength is 15MPa or 15N/mm^2.
Uses: It is used as plain cement concrete PCC for pavement kerbs, floor binding, bedding for footing and in concreting of column, beam and slab as well.

M20 Grade of Concrete:
It comprises mix proportion of 1:1.5:3 (Cement: Sand: Aggregate).
Uses: It is used as Reinforced cement concrete (RCC) in the concreting of structural members like column, slab and beam that are subjected to mild exposure. It is also used in construction of domestic floor and foundations, internal floor slabs, garages and driveways.
Standard Grade of Concrete:
Concrete grade like M30, M35, M40 and M50 are classified as standard grade of concrete and are more strength possessive as compared to lean and ordinary concrete grades.

M25 Grade of Concrete:
Its mix ratio is 1:1:2 (Cement: Sand: Aggregate) with crushing strength of 25MPa.
Uses: It is used as Reinforced Cement Concrete (RCC) for concreting of column, beam and slab of multistorey buildings. It is called Multipurpose concrete mix and used for construction of all areas, mostly it is used in foundations.

M30 Grade of Concrete:
It is used as Reinforced Cement Concrete (RCC) in the construction of pathways, roadways, in concreting of foundation, column, beam and slab of multistorey building.
It is most durable as compared to all above defined grade and thus it is more weather resistant and can take heavier load as compared to other.
Also Read: Curing of Concrete – Methods – Temperature – Stages – Importance

M35 Grade of Concrete:
It is design mix concrete whose proportion is not predetermined like above stated grades, but it can be determined in lab by following standard specifications.
Uses: It is used as Reinforced Cement Concrete (RCC) for the construction of commercial structures, for concreting of beam, slab and column of multistorey building, for concreting external walls, slabs and in structural piling.
Also Read: How to remove paint from concrete? – with or without chemicals

M40 Grade of Concrete:
It is used as RCC in prestressed columns, beams, slabs, footing etc.,
It is also used for construction of runways, concrete roads, pressurized concrete girder, pressurized beams and RCC columns.
It is most durable concrete grade that withstands chemical corrosion therefore it is mostly use in farm where slurry corrode structure or septic tank.
Also Read: Weight of concrete – Density – Concrete Slab – Driveways

M50 Grade of Concrete:
It is also used as RCC concrete.
It is used as per requirement of high strength concrete structure.
It is used in construction of runways, concrete road, pressurized concrete girders, RCC columns.
Also Read: Modulus of Elasticity of Concrete – Static – Dynamic – Types
High Strength Grade of Concrete:

M60 & M80 Grade of concrete:
It is high strength grade of concrete, and it is used in structures where high strength is required such as in long span bridges, high rise buildings, spillways of dam and in coastal construction.