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Concrete compressive strength calculator
Enter two diameters measured at right angles, the length if you want the L/D check, and the maximum load for each cylinder. You get the area, L/D, correction factor and strength for every cylinder, plus the average of the set.
By ProofGrit Editorial TeamLast reviewed:
Results update as you typeUnits: psi, in, lbfRuns in your browser. Nothing is saved.
Key takeaways
- Use the average of two diameters, never the nominal size.
- L/D correction: 1.75 → 0.98, 1.50 → 0.96, 1.25 → 0.93, 1.00 → 0.87, linear between.
- Average cylinders at full precision; round once to the nearest 10 psi.
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How it's calculated
How is the result calculated?
Strength (psi) = max load (lbf) ÷ (π × D² ÷ 4), with D the average of two diameters, corrected when L/D ≤ 1.75.
- 01D = (D1 + D2) ÷ 2, the average of two diameters at right angles
- 02A = π × D² ÷ 4 (in²)
- 03Strength = max load ÷ A (psi), reported to the nearest 10 psi
- 04If L/D ≤ 1.75, multiply by the correction factor: 1.75 → 0.98, 1.50 → 0.96, 1.25 → 0.93, 1.00 → 0.87, interpolating linearly between them
- 05If L/D > 1.75, no correction. If L/D < 1.00, the result is outside the correction range and is not reported
Rounding: strength to the nearest 10 psi. Full precision is kept between steps, and pass/fail uses the displayed value.
FAQ
Frequently asked questions about the concrete strength calculator
Why two diameters?
Cylinders are rarely perfectly round. Averaging two diameters measured at right angles gives a fairer cross-sectional area than a single reading.
Do I have to enter the length?
No. Without a length the calculator reports the uncorrected strength and tells you the L/D correction was not evaluated. Enter the length when a cylinder may be short, for example a cored or trimmed specimen.
What happens when L/D is below 1.00?
The correction table stops at 1.00, so the calculator flags the cylinder as outside the correction range and does not report a strength or include it in an average.
Is the average rounded from rounded values?
No. Each strength is kept at full precision, the average is taken from those values, and only the displayed result is rounded to the nearest 10 psi.
Can I mix 4 × 8 and 6 × 12 cylinders?
The math works for any size, but an average for acceptance should come from cylinders of the same size and age. Use the ACI 318 acceptance calculator for that check.
Related guides and pages
- ASTM C39: compressive strength of concrete cylindersHow a cylinder break works, how strength is calculated from two diameters and the maximum load, when the L/D correction applies, and what to check.
- ASTM C31: making and curing concrete cylinders in the fieldA walk-through of making and curing field cylinders, the initial curing temperature ranges, and how handling errors show up in acceptance results.
- Fresh concrete tests: slump, air, temperature and unit weightA plain-English tour of fresh concrete tests: sampling, slump, air content, temperature and unit weight, and why the field sheet matters later.
- Concrete testing softwareHow ProofGrit handles this test in the lab.
- Standards indexEvery standard explained on this site, with plain-English purpose.
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Sources and further reading
- ASTM C39/C39M, compressive strength of cylindrical concrete specimensASTM International
- ASTM C31/C31M, making and curing concrete test specimens in the fieldASTM International
- American Concrete InstituteACI
Standards are summarized in our own words. Read the current edition from the publisher before you test.
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