Concrete cube test calculation and IS 456 acceptance
This concrete cube test calculator turns the failure loads read off a compression testing machine into compressive strength, averages each set of three cubes, flags any set where one cube is more than 15% away from the average, and checks the results against the acceptance rules of IS 456:2000 Table 11 for the grade you specify.
Enter loads in kN or tonnes, or strengths you have already worked out, for 150 mm or 100 mm cubes, and it shows each step: area, strength of every cube, sample average, validity, and the pass or fail of each check, in N/mm², kg/cm² and psi.
Results are informational. Acceptance or rejection of concrete is decided by the engineer-in-charge under the contract, using a calibrated machine and the current editions of the standards.
How to use the Concrete Cube Strength Calculator
- Choose the grade (M15–M60 or your own fck), the age at test and the cube size.
- Pick whether your readings are loads in kN, loads in tonnes, or strengths in N/mm².
- Enter the three cube readings for each sample in the order the samples were taken. Add or remove samples as needed.
- Leave the standard deviation as assumed (IS 456 Table 8), or enter an established value from 30 or more results.
- Optionally enter the concrete quantity to see the minimum number of samples, then read the verdict and copy the report.
Concrete Cube Strength Calculator features
Load to strength
Strength = load × 1,000 ÷ cube area in mm², for 150 mm, 100 mm or any measured cube size.
±15% validity check
Flags a sample whose cubes vary more than 15% from their average, as IS 456 cl. 15.4 requires.
Table 11 groups of four
Checks each non-overlapping group of four consecutive results and every individual result.
Small pours handled
Uses the Note 2 limits automatically when fewer than four samples are entered.
Early-age readings
7- and 14-day results are shown against commonly quoted strength gains, marked as a guide only.
Concrete cube test formula
Compressive strength is the maximum load divided by the loaded area: fc = P ÷ A. With the load in kN and the cube side in mm, fc (N/mm²) = P × 1,000 ÷ side². A 150 mm cube has an area of 22,500 mm², so a failure load of 675 kN gives 675,000 ÷ 22,500 = 30.0 N/mm². A 100 mm cube failing at 300 kN also gives 30 N/mm².
IS 516:1959 (cl. 5.6) takes the average of three specimens as the result of a sample, provided no specimen varies by more than ±15% from that average; IS 456 cl. 15.4 repeats this and calls such a sample result invalid. IS 516 uses 150 mm cubes, allowing 100 mm cubes when the largest aggregate is 20 mm or smaller, loaded at about 140 kg/cm² (≈ 14 N/mm²) per minute. The 1959 standard has been replaced by IS 516 (Part 1/Sec 1):2021, so check the current edition for the procedure.
IS 456 acceptance criteria (Table 11, Amendment 4)
These are the limits as substituted by Amendment No. 4 (May 2013), which applies the same rule to M15 and above; the original 2000 text had a separate fck + 4 / fck − 4 rule for M20 and above. Without an established standard deviation, Table 8 gives 3.5 N/mm² for M10–M15, 4.0 for M20–M25 and 5.0 for M30–M60, plus 1 N/mm² where site control is poorer than its Note 1 describes. Worked example: M25 with SD 4 needs a group mean of at least 25 + 3.3 = 28.3, rounded to 28.5 N/mm², and every result at least 22 N/mm².
| Check | Requirement (N/mm²) |
|---|---|
| Mean of each group of 4 non-overlapping consecutive results | ≥ fck + 0.825 × SD (rounded to nearest 0.5) or fck + 3, whichever is greater |
| Each individual result | ≥ fck − 3 |
| Up to 30 m³, fewer than 4 samples: mean of all | ≥ fck + 4 |
| Up to 30 m³: each individual result | ≥ fck − 2 |
| Only one sample | ≥ fck + 4 |
Sampling frequency and early-age results
IS 456 cl. 15.2.2 sets the minimum number of samples per grade: 1 for 1–5 m³, 2 for 6–15 m³, 3 for 16–30 m³, 4 for 31–50 m³, and one more for each further 50 m³ or part, with at least one sample per shift. Each sample is three cubes.
Seven-day tests are optional (cl. 15.1.1) and the 28-day strength alone decides acceptance. As a rough guide often quoted for ordinary Portland cement concrete, cubes reach about 40% of their 28-day strength at 3 days, 65% at 7 days and 90% at 14 days. Blended cements, cold weather and poor curing gain strength more slowly.
Planning a pour? The concrete calculator gives volumes and materials, the steel quantity calculator the reinforcement, and the standard deviation calculator helps build an established SD from 30 or more results.
Concrete Cube Strength Calculator FAQ
What is the formula for the concrete cube test?
Compressive strength = failure load ÷ cross-sectional area. For a 150 mm cube, N/mm² = kN × 1,000 ÷ 22,500, i.e. kN ÷ 22.5. 450 kN gives 20 N/mm²; 675 kN gives 30 N/mm².
What is the concrete cube testing procedure as per IS code?
Cubes are cast and cured as IS 516 describes, then crushed at the specified age between the platens on the sides as cast, loaded steadily until failure. Three cubes make one sample, and the result is their average if none differs by more than 15%. Use the current IS 516 for the full procedure.
What should the 7-day strength of M20 or M25 concrete be?
A commonly quoted guide is about 65% of fck at 7 days — roughly 13 N/mm² for M20 and 16–17 N/mm² for M25. IS 456 does not set a 7-day limit; acceptance depends on 28-day results.
How many cubes are needed for a pour?
Each sample is three cubes. IS 456 asks for at least 1 sample for up to 5 m³, rising to 4 samples for 31–50 m³ and one more for each additional 50 m³. The calculator shows the minimum when you enter the quantity.
Why is my sample marked invalid?
One of its three cubes is more than 15% above or below the sample average. IS 456 cl. 15.4 says such a sample’s test result is invalid; check that cube for honeycombing or wrong dimensions.
Which edition of the acceptance criteria does it use?
IS 456:2000 Table 11 as substituted by Amendment No. 4 (May 2013), read on 19 October 2026. If your contract specifies other criteria, such as those for ready-mixed concrete under IS 4926, follow the contract.