Concrete Cylinder Strength Calculator
Turn the failure load of concrete test cylinders or cubes into compressive strength in MPa, psi and kg/cm², average up to three specimens and compare the result with the specified f′c.
How it works
strength = failure load ÷ loaded area · cylinder area = π × d² ÷ 4 · 1 MPa = 145.04 psi = 10.197 kg/cm² · cube → cylinder ≈ × 0.8 · acceptance (ACI 318 §26.12.3): average ≥ f′c and no test below f′c − 3.5 MPa (or 0.9 f′c above 35 MPa)
In the compression test (ASTM C39, EN 12390-3) a cylinder or cube is loaded to failure and the strength is the maximum load divided by the cross-sectional area. Standard US and Latin American practice uses 150 × 300 mm (6 × 12 in) or 100 × 200 mm cylinders; Europe, India and the UK often use 150 mm cubes, which read about 20–25% higher than cylinders of the same concrete, hence the 0.8 conversion. ACI 318 accepts a concrete class when the average of three consecutive tests is at least f′c and no single test (the average of two cylinders) falls more than 3.5 MPa (500 psi) below it. Tests are normally at 28 days; earlier results are lower.
Worked example
Three 150 × 300 mm cylinders fail at 400, 410 and 395 kN. The area is π × 75² = 17,671 mm², so the strengths are 22.64, 23.20 and 22.35 MPa; the average is 22.7 MPa (3,297 psi, 232 kg/cm²), which is 109.8% of an f′c of 20.7 MPa (3,000 psi) — it meets the specification.
Sources: ASTM C39 Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens · ACI 318-19 §26.12.3 — Acceptance criteria for standard-cured specimens
Frequently asked questions
How do you calculate concrete compressive strength?
Divide the maximum load by the specimen area. For a 150 mm cylinder the area is 17,671 mm², so 400 kN ÷ 17,671 mm² = 22.6 MPa (N/mm²).
How do I convert MPa to psi and kg/cm²?
1 MPa = 145.04 psi = 10.197 kg/cm². So 21 MPa ≈ 3,000 psi ≈ 210 kg/cm², and 28 MPa ≈ 4,000 psi ≈ 280 kg/cm².
Why are cube strengths higher than cylinder strengths?
A cube is shorter relative to its width, so friction with the press platens confines it more. Cube results are usually 1.2–1.25 times cylinder results — the Eurocode class C25/30 means 25 MPa on cylinders and 30 MPa on cubes.
What if my cylinders test below f′c?
One low result is not necessarily a failure. ACI 318 looks at the average of three consecutive tests and allows single tests down to f′c − 3.5 MPa; below that, check testing procedures and consider cores from the structure.
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This calculator is for educational purposes. Double-check important results before acting on them.