What does it mean?
Concrete strength classes describe specified compressive strength using a defined test basis. In the European-style C25/30 notation, the two numbers represent characteristic cylinder and cube strengths in MPa, respectively. Shorthand such as C20, C25, C30, C35 or C40 omits part of that designation and should be checked against the specification rather than assumed to be a complete class.
In practice
Common full designations include C20/25, C25/30, C30/37, C35/45 and C40/50. A volume calculator determines how much concrete is needed; it cannot select which strength or exposure specification is appropriate.
Keep in mind
When preparing an estimate, record the specification reference, units and source of each assumption. Separate the net measured quantity from purchase allowances so changes remain easy to trace. Confirm product dimensions, yields and installation requirements with the supplier. Quantities do not establish structural adequacy or material suitability: use the project’s approved design and details, and seek the responsible designer’s clarification where the specification is incomplete.