Concrete testing software
Concrete testing software for small labs
Cylinder strength with the L/D correction, ACI 318 acceptance across every running average, and plain-English guides to C39, C31 and fresh concrete tests. Built for concrete quality control in labs of 1 to 20 people, in psi, inches and pounds-force.
Free calculators. No sign-up. They run in your browser and save nothing.
By ProofGrit Editorial TeamLast reviewed:
Key takeaways
- Strength = max load ÷ (π × D² ÷ 4), with D from two diameters at right angles.
- L/D at or below 1.75 is corrected; below 1.00 is flagged, not reported.
- ACI 318: every running average of three tests ≥ f'c, and no single test below its limit.
- Fresh concrete and curing records explain low breaks before cores are needed.
Calculated live by the library
What does a reviewer see for each test?
- Average diameter
- 6.01 in
- Area
- 28.37 in²
- L/D
- 2.00No correction
- Strength
- 4,230 psi
- 4 × 8 in at 50,000 lbf
- 3,980 psi
- Test 1 average
- 4,125 psi
- Test 2 average
- 3,650 psi
- Test 3 average
- 4,275 psi
- Running average of 3
- 4,017 psipass
- Single-test limit 3,500 psi
- lowest checkedpass
How we handle the math
How does ProofGrit handle the math?
Every calculator on this site uses one tested calculation library. The rules below are the ones it follows.
- 01
Two diameters, always
Area comes from the average of two diameters measured at right angles, never from the nominal size.
- 02
L/D correction, or a clear flag
Short specimens are corrected by interpolation between the table values. Below an L/D of 1.00 the result is flagged, not reported.
- 03
Round once, at the end
Strengths stay at full precision through every average. Only the reported value is rounded to the nearest 10 psi.
- 04
Every running average counts
Acceptance checks each group of three consecutive tests, not just the latest, plus the single-test limit for your f'c.
Free tools
Which free calculators cover this work?
- Concrete compressive strength calculatorCylinder strength from two diameters and the maximum load, with L/D correction and the set average.
- ACI 318 acceptance calculator for concrete strengthCheck strength tests against both ACI 318 acceptance criteria: running average of three and the single-test limit.
Guides
Which guides explain the methods?
- 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.
- CMT lab accreditation: AASHTO R18, ISO/IEC 17025 and recordsWhat AASHTO re:source accreditation, AASHTO R18 and ISO/IEC 17025 mean for a small CMT lab, and the records your lab and its software need to keep.
FAQ
Frequently asked questions
What can I use today?
The concrete compressive strength calculator and the ACI 318 acceptance calculator, plus the guides on this page. They are free and run in your browser.
Are my results saved?
No. The calculators keep nothing. Copy or print the results into your own records.
Does it handle 4 × 8 and 6 × 12 cylinders?
Yes. The strength math works for any size, and the acceptance check applies the cylinder count rule for each size: at least two 6 × 12 or at least three 4 × 8 per test.
Which single-test limit is used?
f'c − 500 psi when f'c is 5,000 psi or less, and 0.90 × f'c above 5,000 psi.
Can I trust the math?
Each formula is shown on the calculator page, and the library is covered by automated tests against known results. You can retrace any number by hand.
Wondering if ProofGrit fits your lab?
Tell us what you test and how many people run it. The team replies by email.