Concrete Cube Test: Procedure, IS Codes, Formula and Acceptance Criteria
November 13, 2026
The concrete cube test is a compression test. A 150 mm cube of concrete is put in a compression testing machine and crushed to find its strength. This is normally done at 28 days. The strength of concrete is given in the design, so we have to check it by testing.
This test is also called the cube compression test, compressive strength test of concrete, and crushing test. All these names refer to the same procedure. The result is useful to the structural engineer, the contractor, and the owner, because it shows that the concrete used on site is the same as the concrete that was designed.
This guide discusses what is cube test for concrete is, why a concrete cube test is carried out, the importance of the concrete cube test, concrete compression tests, and others in detail.
Which IS Codes Apply to the Concrete Cube Test?
There is no single IS code that covers the concrete cube test from start to finish. A combination of multiple concrete cube test standards governs the sampling, making, testing, and structural acceptance of concrete. While IS 516:1959 is the historical standard most people still quote, it was officially withdrawn and replaced by revised parts in 2021. Below is the concrete cube test as per IS codes
The IS Code Map for Concrete Cube Testing
| Standard | Edition | What it governs | What it does not govern |
|---|---|---|---|
| IS 516 (Part 1/Sec 1) | 2021 | Cube testing and calculation of results | Sampling and making/curing specimens |
| IS 1199 (Part 1) | 2018 | Sampling fresh concrete | Making, curing and testing cubes |
| IS 1199 (Part 5) | 2018 | Making and curing test specimens | Sampling and cube testing |
| IS 14858 | 2000 | Compression testing machine requirements | Cube testing procedure |
| IS 10086 | 1982 | Metal cube mould requirements | Compaction and curing |
| IS 456 | 2000 | Sampling frequency and acceptance criteria | Laboratory test procedure |
| IS 4926 | 2003 | RMC quality control and supply | Site and laboratory testing procedures |
IS 516 (Part 1/Sec 1):2021 replaces both IS 516:1959 and IS 5816:1999. The making and curing of specimens is now given in IS 1199 (Part 5):2018 and not in IS 516. So, if someone is still following the 1959 edition, the procedure is now divided into two newer standards.
Many people refer to IS 456 for the test method, but it gives only the sampling frequency and the acceptance. The test method is given in IS 516 and IS 1199.
IS 4031 is also mentioned sometimes with this test, but it is for testing of cement, not for concrete cubes.
Concrete Cube Size and Specimen Requirements
The standard concrete cube test size is 150 mm × 150 mm × 150 mm. A 100 mm cube can also be used, but only when the largest nominal aggregate size is not more than 20 mm.
Cube sizes permitted for the compression test.
| Size | When used | Cross-sectional area | Note |
|---|---|---|---|
| 150 mm | Standard specimen | 22,500 mm² | Use for all aggregate sizes |
| 100 mm | Alternative, only if largest nominal aggregate ≤ 20 mm | 10,000 mm² | A smaller cube reads higher; do not mix sizes in one result |
The 150 mm size is taken as standard because the cube should be big enough, compared to the aggregate size, to represent the concrete properly. Smaller cubes give higher readings, so the results of different sizes cannot be used in place of each other.
Concrete cube test moulds should be of steel or cast iron. They should be watertight and non-absorbent. The dimensions of the moulds must be checked at least once a year. If the mould is distorted, the cube will not be a true cube, and this error will come in the result.
Cube or Cylinder: Why the Numbers Differ
In India, we use cubes, while ASTM and most of Europe use cylinders. The same concrete gives a lower value on a cylinder than on a cube. So, a strength value can be understood only when we know which specimen was used. The cube-to-cylinder ratio is not constant. It changes with the strength level, and IS does not give any fixed ratio. Indian grades like M25 are based on cubes. So, when you read American or European material, take their values as a different measurement and do not convert them using any thumb rule.
Apparatus Required for the Cube Test
The concrete cube testing machine (CTM) applies an increasing load on the cube at a controlled rate till the cube fails. It records the maximum load.
Apparatus checklist for the cube test
| Item | Specification | Purpose | Standard |
|---|---|---|---|
| CTM | Calibrated machine | Crushes cube & records peak load | IS 14858 |
| Moulds | Watertight steel/cast iron (check annually) | Shapes the cube | IS 10086 |
| Tamping Rod | 16 mm diameter, 600 mm long | Manual compaction | IS 1199 (Pt 5) |
| Vibrating Table | Minimum 40 Hz | Mechanical compaction | IS 1199 (Pt 5) |
| Internal Vibrator | Minimum 120 Hz | Mechanical compaction | IS 1199 (Pt 5) |
| Weighing Balance | Suitable for cube mass | Measures mass before crushing | IS 516 |
| Curing Tank | Clean water at 27 ± 2 °C | Cures cubes until testing | IS 1199 (Pt 5) |
Calibration is very important because the machine is the measuring instrument. If the machine is not calibrated, all the results are invalid, even if the cubes were made carefully.
How Many Cubes to Cast and How Often to Sample?
The sampling frequency depends on the quantity of concrete placed. It does not depend on the number of trucks.
Minimum sampling frequency by quantity of concrete placed.
| Quantity placed | Minimum samples | Cubes cast |
|---|---|---|
| 1–5 m³ | 1 | 3 |
| 6–15 m³ | 2 | 6 |
| 16–30 m³ | 3 | 9 |
| 31–50 m³ | 4 | 12 |
| Above 50 m³ | 4, plus 1 for each further 50 m³ | 12, plus 3 per extra sample |
Note: The following table shows the general sampling frequency by concrete quantity for cube-test sampling. For ready-mixed concrete supplied under IS 4926, the applicable RMC sampling requirement is one sample for every 50 m³ or every 50 batches, whichever gives more samples, with at least one representative sample from each work shift. Project specifications or contractual requirements may prescribe additional sampling.
One sample means three cubes. This is the basis for determining the number of concrete test cubes required for each sample.
Many sites cast more than three cubes per sample so that a 7-day result is also available. This is a practice followed at sites and is not a code requirement.
Before the first pour, it should be decided in the contract whether the supplier or the contractor will cast the cubes.
What Is the Concrete Cube Test Procedure, Step by Step?
The procedure is done in four stages, and two standards are followed. The procedure uses IS 1199 (Part 1) for sampling, IS 1199 (Part 5) for making and curing the specimens, and IS 516 (Part 1/Sec 1) for testing.
Step 1: Sampling the Fresh Concrete
- Take a representative composite sample from the point of placement.
- Do not take the sample from the first or the last part of the discharge.
- Governing standard: IS 1199 (Part 1).
Step 2: Filling and Compacting the Moulds
- Fill the mould in three equal layers. For a 150 mm cube, each layer should be approximately 50 mm deep.
- Compact each layer with the 16 mm rod, or on a vibrating table.
- Do not over-vibrate, because it segregates the mix. Under-compaction is the most common cause of a low result.
- Governing standard: IS 1199 (Part 5).
Compaction requirements by cube size
| Cube size | Strokes per layer (16 mm rod) | Alternative |
|---|---|---|
| 150 mm | Minimum 35 | Vibrating table |
| 100 mm | Minimum 25 | Vibrating table |
Step 3: Finishing, Marking and Demoulding
- Strike the top level with a trowel or float.
- Mark each cube in a durable way and keep the records, so that it can be traced back to its pour.
- Demould after at least 16 hours and not more than three days.
- Governing standard: IS 1199 (Part 5).
Step 4: Testing in the Compression Machine
- The recorded weight can also be used when a concrete cube density test is required.
- Place the cube in such a way that the load is applied perpendicular to the direction of casting, never on the trowelled face.
- Centre the cube on the platen.
- Apply the load without shock at the rate of loading for concrete cube test of 14 N/mm² per minute (about 140 kg/cm² per minute) until the cube can take no more load.
- Record the maximum load.
- Governing standard: IS 516 (Part 1/Sec 1).
Curing the Test Cubes
If a cube is not cured properly, the result will show the problem in curing and not the actual quality of the concrete.
Keep the cubes in the mould for at least 16 hours and not more than three days from the time water was added to the mix. After that, put them in clean fresh water at 27 ± 2 °C and take them out just before testing. The temperature is controlled because the speed of curing, and so the strength, changes with temperature.
If the cubes are being sent to an outside laboratory, cover them with wet cloth, wet sand or sawdust, or seal them in plastic bags with water. They should reach the laboratory in damp condition at least 24 hours before the test.
Curing of test cubes is different from curing of the structure. The duration, purpose and code clause are all different.
What invalidates a cube:
- Curing water outside 27 ± 2 °C
- Cubes cured in air instead of water
- Cubes allowed to dry out between the tank and the machine
- Demoulding outside the 16-hour to three-day window
When Cubes Are Tested: 7-Day, 14-Day and 28-Day Results
28 days is the standard age used for concrete acceptance. Tests done before this provide an early indication of strength development.
Test ages, what each decides, and their status under IS
| Age | Primary Purpose | IS Code Status |
|---|---|---|
| 7 Days | Early indicator | No pass mark (early warning) |
| 14 Days | Progress check | No pass mark (growth check) |
| 28 Days | Final Decision | Official Acceptance Age (acceptance check) |
| > 28 Days | Special cases | Only if specified |
Age is counted from the time of casting. Testing much after 28 days does not change the acceptance, because the criteria are given for 28 days.
A 7-day result is an early indication of strength development. For Aparna RMC, a 7-day cube may be expected to achieve around 65% to 70% of the target 28-day characteristic strength (fck). This is an early indicator and is not a final criterion for structural acceptance. If the 7-day result is low, curing, handling and testing conditions should be checked.
Concrete Cube Test Formula and Worked Calculation
Formula: Compressive strength = failure load ÷ cross-sectional area. Use 22,500 mm² for a 150 mm cube and 10,000 mm² for a 100 mm cube. N/mm² and MPa are the same unit. Some Indian site records may also use kgf/cm², so the relevant unit conversion is shown below.
Unit conversions
| From | To | Factor |
|---|---|---|
| 1 N/mm² | MPa | 1 (identical) |
| 1 kgf/cm² | N/mm² | ≈ 0.098 |
| 1 N/mm² | kgf/cm² | ≈ 10.2 |
| kN | N | × 1,000 |
Worked example: three 150 mm cubes from one M25 sample
- Failure loads: 596 kN, 630 kN and 614 kN.
- Individual strengths: 596,000 ÷ 22,500 = 26.49; 630,000 ÷ 22,500 = 28.00; 614,000 ÷ 22,500 = 27.29 N/mm².
- Average: 27.26 N/mm².
- Largest deviation from the average: about 0.77 N/mm², or roughly 2.8%.
- This is inside ±15%, so the sample is valid.
- Illustrative sample result: 27.26 N/mm².
The three cubes together give one result. The concrete cube test calculation is based on the individual cube strengths and their average. It is the average of the three specimens, provided none of them differs from the average by more than ±15%. This result is taken to the next section for checking.
How a Cube Test Result Is Judged?
In most of the published material, two different questions are mixed up. The ±15% rule tells whether a sample is valid. IS 456 tells whether the concrete is accepted. The questions are different, and so the rules are also different.
Acceptance logic
| Criterion | Requirement |
|---|---|
| Sample validity | No individual cube should differ from the 3-cube average by more than ±15%. |
| Group acceptance — M20 and above | Mean of four consecutive, non-overlapping test results ≥ fck + 0.825σ (rounded off to the nearest 0.5 N/mm²) or fck + 4 N/mm², whichever is greater. |
| Individual result — M20 and above | No individual test result should be below fck − 4 N/mm². |
For structural concrete of grade M20 and above, both the group mean and individual-result criteria should be satisfied.
One low cube does not mean rejection. Acceptance is decided on groups of consecutive results and not on one specimen. In our example, the 27.26 N/mm² result for M25 is one test result, and it will be one of the four consecutive results used for the group check. The final decision is taken by the structural engineer.
What to Do When a Cube Test Fails
A low or non-conforming cube result does not automatically mean that the concrete in the structure has failed.
- Check the test first: Check the sampling point, compaction, curing temperature, demoulding time, loading direction, and machine calibration.
- Review the other results: Acceptance is based on consecutive test results, so one low result should be considered along with the other results. Casting or testing additional cubes from the same sample does not retrospectively change the original test result.
- Assess the concrete in place if needed. Core testing and non-destructive methods can provide information about the concrete in the structure. Core tests assess concrete taken from the member, while non-destructive methods provide indirect indications. A rebound hammer reading is not an acceptance test.
- Leave the decision to the structural engineer: The supplier or contractor should not make the final decision on whether the concrete is accepted.
Why Cube Results Go Wrong: Test Accuracy
The causes given below are mistakes in the test and not defects in the concrete.
Common test errors, their effect on the result, and prevention.
| Cause | What it does to the result | How to prevent it |
|---|---|---|
| Sampling from the first or last discharge | Sample is not representative | Take a composite sample at the point of placing |
| Under-compaction | Voids lower the strength reading | Use the stroke count or vibrating table properly |
| Over-vibration | Segregation, uneven cube | Stop vibrating when air bubbles cease |
| Distorted, dirty or unchecked mould | Wrong geometry, wrong area | Check dimensions at least annually; clean before use |
| Curing water off temperature | Strength gain differs from expected | Hold 27 ± 2 °C |
| Cubes cured in air | Under-hydration, low result | Submerge until the test |
| Delay between tank and machine | Drying alters the reading | Test within two hours of removal |
| Loading the trowelled face | Uneven contact, wrong reading | Load perpendicular to the direction of casting |
| Loading too fast | Overstated strength | Keep a constant 14 N/mm² per minute |
| Uncalibrated machine | Every result unreliable | Calibrate on schedule |
| Untraceable specimens | Result cannot be tied to a pour | Mark durably and keep records |
What a Concrete Cube Test Report Must Contain?
A good test report should show where the concrete came from, when it was sampled, how it was cured, its weight, the load at which it failed, and who signed it.
Fields to include:
- Project, location, and element cast
- Supplier or plant, and delivery or batch reference
- Grade and mix reference
- Date and time of sampling
- Specimen identification mark
- Cube size
- Date of casting and date of demoulding
- Curing method and water temperature
- Age at test and date of testing
- Dimensions and weight before crushing
- Failure load
- Compressive strength
- Average, validity check, and reported result
- Name and signature of tester and reviewer
The identification mark should be clear and long-lasting, and the records should show the identity of the specimen from sampling till testing. Weight is recorded before testing, so a proper report has a weight column, which most online templates do not have. Please check that any report from a supplier can be traced back to the pour.
Sample record (M25, 150 mm cubes):
| Cube | Weight | Failure load | Calculated strength |
|---|---|---|---|
| A | Recorded before testing | 596 kN | 26.49 N/mm² |
| B | Recorded before testing | 630 kN | 28.0 N/mm² |
| C | Recorded before testing | 614 kN | 27.29 N/mm² |
| Average | — | — | 27.26 N/mm² |
| Reported result | — | — | 27.26 N/mm² |
Note: This is an illustrative calculation using sample failure loads. It is not an actual laboratory test report or acceptance result.
How Aparna RMC Tests Every Batch?
Cubes are cast from the same concrete that is delivered, and not from a separate mix. Aparna RMC cubes are stored at the casting site for the first 24 hours under damp cover. They are then demoulded and transferred to fully submerged, temperature-controlled water curing tanks maintained at 27 ± 2°C. Testing is carried out on a calibrated compression testing machine at the plant’s on-site quality control laboratory. Cube-test reports are typically ready within 24 hours of the designated crushing time. For example, a 7-day test report is typically available by Day 8, while a 28-day test report is typically available by Day 29.
Aparna RMC’s quality system is certified to ISO 9001:2015, ISO 45001:2018 and ISO 14001:2015. A laboratory tests the concrete and its ingredients and supports the plants. Trial mixes are also made and tested.
If you require RMC for your project, get in touch with Aparna RMC for your requirement. Aparna RMC conducts strict concrete cube tests, along with comprehensive fresh and hardened concrete assessments to ensure your mix matches the required engineering specifications.
Frequently Asked Questions On Concrete Cube Test
What is the 7-day cube test for concrete?
A 7-day cube test checks the early strength of concrete. It gives an idea of how the concrete is gaining strength. However, it is not the final acceptance test. The 28-day cube test is used for the final strength check.
Is IS 516 the code for the concrete cube test?
Yes. IS 516 covers the testing of concrete cubes and calculation of compressive strength. Other codes are also used for sampling, making and curing the cubes, and for acceptance of concrete.
Is IS 456 used for concrete cube sampling and testing?
IS 456 gives requirements for sampling frequency and acceptance of concrete. The actual procedure for making and testing cubes is covered under IS 1199 and IS 516.
How many test cubes are needed per cubic metre of concrete?
There is no fixed number of cubes for every cubic metre. The number depends on the quantity of concrete and the applicable sampling requirement. One sample normally consists of three cubes. For RMC, IS 4926 gives a separate sampling requirement based on volume or number of batches.
How do you calculate 7-day cube strength?
Divide the load at which the cube fails by its cross-sectional area. For a 150 mm cube, the area is 22,500 mm². The answer is the compressive strength in N/mm² or MPa.
How much does a concrete cube test cost?
The cost depends on the number of cubes, who conducts the test and whether an outside laboratory is used. The supplier and contractor should decide the testing responsibility before the concrete work starts.
Can 100 mm cubes be used instead of 150 mm?
Yes. A 100 mm cube can be used when the largest nominal aggregate size is not more than 20 mm. A 150 mm cube is the standard size. Results from different cube sizes should not be mixed.
What if one cube in a set of three is much lower than the other two?
First check the average and the allowed variation. Sampling, compaction, curing and testing should also be checked. One low cube does not automatically mean that the concrete has failed.
How long can a cube stay out of the curing tank before it is tested?
A cube should normally be tested within two hours after it is removed from the curing tank. It should not be allowed to dry before testing, as this can affect the result.
Who is responsible for casting and curing cubes — the supplier or the contractor?
It depends on the project agreement. The supplier or contractor can be responsible for casting and curing the cubes. This should be decided before the first pour and recorded clearly.