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ACI 318 acceptance calculator for concrete strength
Enter the specified strength, the cylinder size and the cylinder strengths for each test. The calculator averages each test, builds every running average of three consecutive tests, and checks both acceptance criteria with a plain explanation.
By ProofGrit Editorial TeamLast reviewed:
Results update as you typeUnits: psiRuns in your browser. Nothing is saved.
Key takeaways
- A strength test averages at least two 6 × 12 in or three 4 × 8 in cylinders at one age.
- Every overlapping group of three consecutive tests is checked, not only the latest.
- With fewer than three tests, the running-average check stays pending.
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How it's calculated
How is the result calculated?
Pass when every average of 3 consecutive tests ≥ f'c, and no test is below f'c − 500 psi (f'c ≤ 5,000) or 0.90 × f'c (f'c > 5,000).
- 01One strength test = average of at least two 6 × 12 in cylinders or at least three 4 × 8 in cylinders tested at the same age (usually 28 days)
- 02Criterion (a): every average of 3 consecutive strength tests must be at least f'c
- 03Criterion (b): no single test below f'c − 500 psi when f'c ≤ 5,000 psi, or below 0.90 × f'c when f'c > 5,000 psi
- 04With fewer than 3 tests, criterion (a) stays pending rather than passing
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 aci 318 acceptance calculator
Why does the result say pending?
The running-average criterion needs three consecutive strength tests. Until you have three, the calculator shows that check as pending instead of pretending it passed.
Does every running average have to pass?
Yes. The calculator checks each overlapping group of three, not just the most recent one, and shows each group in the table.
Why is the single-test limit different above 5,000 psi?
For f'c up to 5,000 psi the limit is f'c minus 500 psi. Above 5,000 psi it becomes 90 percent of f'c, which is the stricter of the two at higher strengths.
Can I enter tests at different ages?
No. An acceptance series uses tests at the same age, so one age field applies to every test. Check other ages separately.
Is this a substitute for the engineer's evaluation?
No. It shows the arithmetic of the acceptance criteria. Low results still need the investigation and judgment your project specifications call for.
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
- American Concrete InstituteACI
- ASTM C39/C39M, compressive strength of cylindrical concrete specimensASTM International
- ASTM C31/C31M, making and curing concrete test specimens in the fieldASTM International
Standards are summarized in our own words. Read the current edition from the publisher before you test.
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