Reference
Concrete grades compared
M25, C20/25 and 2900 psi describe the same concrete. M25 and 25 MPa cylinder strength do not. What each standard actually measures, and how the grade ladders line up against each other.
Written againstIS 456EN 206ACI 318
A cube and a cylinder do not read the same
Concrete has no single compressive strength; it has the strength that a particular specimen, crushed a particular way, reports. India and the United Kingdom crush a 150 mm cube. The United States and mainland Europe crush a 150 × 300 mm cylinder. The same batch gives a lower number as a cylinder.
The reason is friction. The press platens grip the ends of the specimen and stop them spreading sideways, which holds the concrete together and lets it carry more load. A cube is short enough that this restraint reaches most of it. A cylinder is twice as tall as it is wide, so its middle is far enough from both platens to fail unconfined — and it fails earlier.
So an IS 456 grade and an ACI f'c are not interchangeable numbers, and reading M25 as 25 MPa on an American drawing specifies concrete about a quarter stronger than intended.
The grade ladders
IS 456 names a grade for its characteristic cube strength: M25 is a 25 N/mm² cube at 28 days, at the 5% fractile — meaning up to one test in twenty is permitted to fall below it. EN 206 names both figures at once, cylinder first: C20/25 is the same concrete. ACI 318 quotes the cylinder alone, as f’c.
The two ladders are shown merged on cube strength, and they do not line up. IS has M35 and M40; EN steps from C25/30 straight to C30/37 and then to C35/45, with nothing at cube 35 or 40. EN has C30/37; IS has nothing at 37. Those rows are left with gaps rather than filled in, which is the honest shape of the comparison.
| IS 456 | EN 206 | CylinderN/mm² | CubeN/mm² | f’cpsi | Cyl / cube |
|---|---|---|---|---|---|
| M10 | C8/10 | 8 | 10 | 1160 | 0.800 |
| M15 | C12/15 | 12 | 15 | 1740 | 0.800 |
| M20 | C16/20 | 16 | 20 | 2320 | 0.800 |
| M25 | C20/25 | 20 | 25 | 2900 | 0.800 |
| M30 | C25/30 | 25 | 30 | 3630 | 0.833 |
| M35 | — | — | 35 | — | — |
| — | C30/37 | 30 | 37 | 4350 | 0.811 |
| M40 | — | — | 40 | — | — |
| M45 | C35/45 | 35 | 45 | 5080 | 0.778 |
| M50 | C40/50 | 40 | 50 | 5800 | 0.800 |
| M55 | C45/55 | 45 | 55 | 6530 | 0.818 |
| M60 | C50/60 | 50 | 60 | 7250 | 0.833 |
Nominal mixes, and where they stop
IS 456 gives proportions by volume for low grades, for work small enough that batching by weight is not practical. The table is short, and where it ends is the important part.
| Grade | 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 |
Which grade to specify
This page compares grades; it does not choose one. That decision belongs to the engineer of record, and it is not made from strength alone — IS 456 sets minimum grades by exposure condition, and for reinforced concrete in a marine or coastal environment the durability requirement, not the load, is usually what fixes the grade.
If you are reading this to check a number on a drawing that someone else prepared, that is what it is for. If you are reading it to decide a grade yourself, the standard is the document you need, not this table.