Concrete Strength Converter
Convert between grade, MPa, psi and kgf/cm², with cube-to-cylinder correction.
Quick answer
With the default inputs: Grade C30 · Specimen 150 mm cube · Measured strength 0 MPaCube strength 30.0 MPa
Edit the inputs below to recalculate.Inputs
Result
Cube strength
30.0MPa
| Cylinder strength | 23.7MPa |
| psi (cylinder) | 3437psi |
| psi (cube) | 4351psi |
| kgf/cm² | 305.9kgf/cm² |
| Design compressive fc | 14.3MPa |
| Design tensile ft | 1.43MPa |
| Elastic modulus Ec | 2.98×10⁴ MPa |
| US equivalent (rounded) | 3500psi |
The 0.79 cube-to-cylinder factor (up to C50) follows GB/T 50081 and international practice. It is for comparison only and does not replace a test report.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
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.
How to work it out
- Confirm the specimen shape first: a 150 mm cube or a Φ150×300 cylinder. Getting this wrong invalidates everything downstream.
- Cube to cylinder × 0.79 up to C50; above C60 the ratio climbs to 0.83-0.86, so stop using 0.79 there.
- To psi × 145.038 (1 MPa = 145.038 psi); to kgf/cm² × 10.197.
- A grade means the characteristic cube strength: C30 is fcu,k = 30 MPa on 150 mm cubes at 28 days with 95% probability of being exceeded.
- For modulus, estimate Ec = 10⁵ ÷ (2.2 + 34.7/fcu) in MPa.
Where it goes wrong
- Not distinguishing cube from cylinder — 38 MPa on cubes is about 30 MPa on cylinders, exactly one grade apart; confusing them passes failing concrete.
- Reading C30 as 'must reach 30 MPa' — the characteristic value allows 5% below it. Compliance is judged statistically per GB/T 50107; one low result is not automatically a failure.
- Mixing psi and ksi — 4000 psi is about 27.6 MPa, while 4000 ksi is a thousand times out.
Material and cost
Cube testing runs 50-120 per set of three; rebound testing 300-800 per element; core drilling 400-800 per core. Remediating or demolishing failing concrete costs six figures, so testing is never the place to save.
On site
- Standard-cured specimens are for strength assessment; field-cured specimens are for striking formwork and in-situ testing. They are not interchangeable.
- Non-150 mm cubes need size factors: 100 mm × 0.95, 200 mm × 1.05.
- Sampling frequency per GB 50204: one set per 100 m³ and per work shift, with a set for any partial batch.
Basis
- GB/T 50081-2019 Standard for test methods of concrete physical and mechanical properties
- GB/T 50107-2010 Standard for evaluation of concrete compressive strength (acceptance rules)
- GB 50010-2010 Code for design of concrete structures, Section 4.1 (characteristic strengths and modulus)
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.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Concrete Strength Converter. 2026-10-25. https://jianzhujisuan.com/en/concrete-strength-convert
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.