Guide
Concrete Grade Equivalents: M, C and psi
An Indian drawing says M25. A European one says C20/25. An American one says 3000 psi. They are describing the same material through three different tests, and translating between them is mostly straightforward — except for the two places where the usual conversion chart is simply wrong.
The difference is the specimen, not the concrete
Grade is characteristic compressive strength: the value 95% of test results should exceed. What varies between systems is the shape of the thing tested.
India and Britain crush a cube. Most of Europe and the United States crush a cylinder. The same concrete gives a higher number as a cube, because the steel platens grip the cube's full width and restrain it sideways, which holds it together and lets it take more load before failing. A cylinder is slimmer relative to its height and gets less of that help.
So M25 means 25 N/mm² on a cube. C20/25 states both: 20 on a cylinder, 25 on a cube — the same concrete, both tests printed. Once you see that, the European designation stops being cryptic and becomes the most useful of the three, because it is the only one that shows its working.
The 0.80 rule is not a rule
Almost every conversion page prints the same shortcut: cylinder strength equals 0.80 times cube strength. It is a reasonable average and it is not what the standard says.
EN 206's own paired grades give ratios from 0.778 to 0.833. C35/45 is 0.778. C25/30 and C50/60 are both 0.833. Several grades — C16/20, C20/25, C40/50 — do land on exactly 0.80, which is why the shortcut survives.
The relationship drifts with strength because stronger concrete is more brittle and gains less from the platen restraint. So no single factor is correct across the range, and applying 0.80 to a grade the standard already pairs differently means overriding the standard with an approximation of itself.
The honest approach is to read both numbers off the EN designation rather than converting at all.
M35 and M40 have no European equivalent
This is the part conversion charts invent.
EN 206 has no grade at 35 N/mm² cube and none at 40. It steps C30/37, then C35/45. IS 456 has M35 and M40. The ladders simply do not line up in that range, and there is no C-grade to pair with either.
Charts that print "M35 = C30/37" are matching on proximity, not equivalence. C30/37 is 37 N/mm² on a cube; M35 is 35. Specifying one where the drawing called for the other is a 2 N/mm² substitution somebody made silently on your behalf.
The same gap runs the other way: C30/37 has no IS partner. If a drawing specifies it and your supplier works in M-grades, that is a conversation to have rather than a lookup to perform.
Reading a drawing you did not write
Take the designation at face value and find out which test it refers to. A bare number in N/mm² on an Indian or British drawing is a cube. On a European or American one, check — psi is a cylinder, and a European C-grade tells you both.
If you need to compare across systems, compare cube to cube or cylinder to cylinder. Do not convert one to the other with a factor unless you have no choice, and if you must, say which factor you used.
And where the ladders have no matching rung — M35, M40, C30/37 — do not fill the gap with the nearest neighbour. Ask.
Common questions
What is the difference between M25 and C25/30?
They are not the same. M25 is 25 N/mm² on a cube. C25/30 is 25 on a cylinder and 30 on a cube — a stronger concrete. The M-grade that pairs with C20/25 is M25; the one that pairs with C25/30 is M30.
Concrete grades compared →Is cylinder strength really 0.80 of cube strength?
Often, but not always. The standard's own pairs range from 0.778 at C35/45 to 0.833 at C25/30 and C50/60. Several grades are exactly 0.80, which is why the shortcut persists. Read both numbers off the EN designation instead of converting.
What is M35 in European grades?
Nothing. EN 206 has no grade at 35 N/mm² cube — it goes C30/37 then C35/45. Charts pairing M35 with C30/37 are matching on proximity and quietly substituting a 37 for a 35.
What is the mix ratio for M25?
IS 456 does not give one. Nominal mixes stop at M20 at 1 : 1.5 : 3; above that the code requires a design mix, because proportioning by volume stops being reliable at higher strengths. Any page printing a nominal ratio for M25 invented it.
Concrete volume calculator →How do I convert a grade to psi?
Multiply the cylinder strength by 145. M25 pairs with C20/25, so 20 N/mm² is about 2900 psi. Use the cylinder figure, not the cube one — American practice specifies f'c on cylinders, so converting from the cube number overstates it by around a fifth.