Concrete Strength Converter
Convert between grade, MPa, psi and kgf/cm², with cube-to-cylinder correction.
Inputs
Result
Formula
| Cylinder strength | f_cyl = 0.79 × f_cube (up to C50) |
| psi | 1 MPa = 145.038 psi |
| kgf/cm² | 1 MPa = 10.197 kgf/cm² |
| Elastic modulus | Ec = 10⁵ ÷ (2.2 + 34.7/fcu) MPa |
C30: 30.0 MPa cube strength — 23.7 MPa as a cylinder (3438 psi, or 4351 psi as a cube) and 306 kgf/cm² — with fc = 14.3 MPa, ft = 1.43 MPa and Ec = 2.98×10⁴ MPa.
FAQ
Does C30 mean 30 MPa?
C30 means a characteristic cube strength of 30 MPa at 28 days under standard curing, with 95% confidence. That is not the design value: the design compressive strength of C30 is only 14.3 MPa after the material partial factor and the prism-to-cube correction. Designing with 30 MPa would be badly unsafe.
Why convert between cube and cylinder?
A cube benefits less from platen restraint and reads higher; a cylinder is closer to the stress state in a real member, so the same concrete tests about 21% higher as a cube. China and Europe use cubes, the US and Japan use cylinders — convert when reading foreign drawings or you will misread the grade.
Can a test result be converted back to a grade?
Only with the age and curing conditions stated. Direct comparison requires 28-day standard curing; site-cured or early-age specimens must first be normalised to 28 days by maturity or an accepted curve. A single failing result does not condemn the concrete — the code allows cores taken from the structure.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.