Concrete Strength Classes (EN 206)
Under EN 206, concrete strength is specified by a class such as C25/30 — the class sets a floor on strength, but the exposure class often sets the mix through its own durability limits.
Reading the notation
A strength class is written C(cylinder)/(cube) — for example C25/30 means a characteristic strength of 25 N/mm² measured on a cylinder, and 30 N/mm² measured on a cube, at 28 days. The target mean strength used in mix design, fcm, is set above the characteristic strength fck by a margin of about 8 N/mm² to allow for normal variability, so a design is not made right at the characteristic limit.
Choose a class
Classes at a glance
| Class | Typical uses |
|---|---|
| C20/25 | Mass concrete, blinding, lightly loaded elements |
| C25/30 | General reinforced concrete |
| C30/37 | Common for reinforced concrete in carbonation / moderate exposure |
| C35/45 | More severe exposure — chlorides, freeze-thaw |
| C40/50 | Higher strength and durability requirements |
| C45/55 | Higher strength and durability requirements |
| C50/60 | Higher strength and durability requirements |
The strength class is only part of the picture: the project’s exposure class (carbonation, chlorides, freeze-thaw, chemical attack) sets its own durability limiting values, and those can require a mix stronger or richer than the strength class alone implies. See the method page for how the two are combined.
Strength classes across European countries
The C-notation is shared across all countries that use EN 206, so a C30/37 in Germany, France or the Netherlands has the same characteristic strengths. What differs nationally is how the class is linked to exposure (the minimum strength class for each exposure class), which test specimens are used for conformity and how specimens are stored. Eurocode 2 structural design uses the characteristic cylinder strength fck — the first number in the class.
Cubes, cylinders and storage conditions
EN 12390-1 permits both 150 mm cubes and 150×300 mm cylinders. France and several southern European countries predominantly test cylinders; Germany and several central European countries predominantly test 150 mm cubes. National provisions may also permit storage methods other than the EN 12390-2 reference water storage, with a conversion to the reference value. When a test report is compared with the specified class, confirm the specimen type and storage method first.
Typical classes by exposure across Europe
As an orientation only: C20/25–C25/30 for internal dry concrete and unreinforced foundations; C25/30–C30/37 for general reinforced concrete exposed to carbonation (XC); C30/37–C35/45 where chlorides (XD, XS) or freeze–thaw with de-icing salt (XF) apply; and higher classes for precast, prestressed and high-performance work. The binding values come from the national complement to EN 206 for the country of use.
Frequently asked questions
What does C25/30 mean?
The first number, 25, is the characteristic cylinder strength in N/mm²; the second, 30, is the characteristic cube strength in N/mm². Both describe the same concrete, tested on different specimen shapes.
Does the strength class alone decide the mix?
No. EN 206 also sets durability limiting values by exposure class — maximum water-cement ratio, minimum cement content, minimum strength class and, where relevant, minimum air content. The more demanding of strength or exposure governs.