Concrete is specified by what it must resist in compression at 28 days, but the two major code families write that number differently. American practice uses a single value, the specified compressive strength f′c from cylinder tests, quoted in psi — ready-mix suppliers stock the familiar ladder from the ACI 318 structural minimum of 2,500 psi through the common residential 3,000 psi order up to 8,000 psi high-strength mixes. A European class like C25/30 names two numbers at once: the characteristic cylinder strength fck (25 MPa) and the characteristic cube strength (30 MPa), both defined so 95% of tests are expected to pass.
The cylinder/cube split is a real physical difference, not a rounding: cubes test roughly 20–25% higher than cylinders because the platens confine the shorter specimen more, and Eurocode design formulas use the cylinder value. That makes cross-system comparison a known trap — a 4,000 psi mix converts to 27.6 MPa and sits between C25/30 and C30/37 by cylinder strength, but matching it against the cube number in the class name would understate the European mix by a full class.
The characteristic strength also anchors the other design properties. EN 1992-1-1 Table 3.1 derives the mean strength fcm = fck + 8 MPa, the tensile strength fctm = 0.30·fck^(2/3), and the secant modulus Ecm = 22·(fcm/10)^0.3 GPa straight from fck — capacity calculations use fck reduced by a partial factor, while producers target the higher mean so no more than 5% of results fall short.