The concrete strength class of normal concrete is specified by two values: the characteristic value for cylinder and cube, each in N/mm². Example: C30/37 means 30 N/mm² cylinder strength, 37 N/mm² cube strength.
Common classes: C20/25 (residential construction), C30/37 (load-bearing structures), C50/60 (prestressed concrete, high-rise buildings). For UHPC classes exist up to C200. The concrete strength class is determined by the structural engineer depending on the exposure class.