About Compressive Strength of Concrete Calculation
This calculator estimates the compressive strength of concrete based on age, average curing temperature, and the selected concrete class. It is suitable for a preliminary assessment of strength development in cast-in-place concrete structures when you need to understand what share of the design strength the concrete may have reached by a given time.
The calculation is useful for an approximate evaluation of formwork removal timing, intermediate control of the curing process, and comparison of the effect of temperature on strength development. The result is shown as a percentage and also as estimated cylinder and cube strength in MPa.
Reference points and recommendations
Calculation model
Calculation basis. The calculator uses the concrete class according to EN 206 and links it to characteristic compressive strength. For the selected class, the characteristic cylinder strength fck and the corresponding cube strength are used. For example, for class C25/30, the values fck = 25 MPa and cube strength 30 MPa are used.
Temperature range. The calculation is performed for an average curing temperature from 0 to 70 °C. This means the calculation uses not the instantaneous air temperature, but the average temperature of the actual curing conditions during the specified period.
Calculation sequence
Step 1. Based on the entered temperature, a temperature coefficient for strength development is selected. Inside the calculator, each whole temperature value from 0 to 70 °C has its own coefficient reflecting acceleration or slowing of curing.
Step 2. A class adjustment coefficient is applied for the selected concrete class. The following values are used in the calculation: 0.94 for C12/15, 0.96 for C16/20, 0.98 for C20/25, 1.00 for C25/30, 1.03 for C30/37, 1.06 for C35/45, and 1.09 for C40/50.
Step 3. The percentage of strength gain is determined by a logarithmic relationship based on concrete age t in days, the temperature coefficient, and the class coefficient. The following expression is used in the calculation:
P = R(T) × ln(t × k + 1) / ln(28 × k)
Here, P is the strength gain in percent, R(T) is the coefficient for temperature T, t is the concrete age in days, and k is the coefficient for the selected concrete class. The meaning of this formula is that strength increases quickly at early ages, and then the rate of increase gradually slows down.
Step 4. The obtained value is limited to a range from 0 to 100 %. This means the calculator does not show negative strength and does not allow the final result to exceed the conditional level of 100 %.
How the final results are formed
Strength gain, %. This is the share of the class strength that the concrete has approximately reached at the specified age and temperature. This value is the basis for all other results.
Estimated cylinder strength. It is calculated using the formula:
f = fck × P / 100
Here, fck is the class cylinder strength in MPa, and P is the calculated strength gain in percent. For class C25/30 with a strength gain of 72 %, the result will be 25 × 0.72 = 18.0 MPa.
Estimated cube strength. It is determined in the same way, but the class cube strength is used instead of the class cylinder strength:
fcube = fcube,class × P / 100
This output is convenient if, on site or in documentation, strength is more often compared using cubes rather than cylinders.
Graph logic
Curve generation. The graph is calculated from 0 days to the greater of two values: the actual concrete age or 28 days. This makes it possible to see not only the current point, but also the approximate strength development curve over the standard curing interval.
Why the 28-day point is used. In European practice, an age of 28 days is generally taken as the basic reference period for comparison with the concrete strength class according to EN 206 and with the design assumptions for structural use according to EN 1992-1-1 Eurocode 2. Design of concrete structures.
Practical interpretation of results
Early-age strength. At short curing times, temperature has a particularly strong effect on the result. For the same concrete class, the difference between curing at 5 °C and 20 °C during the first few days can be substantial, so it is important to use the average temperature for the period when comparing scenarios.
Final value. If you need an approximate assessment of whether the concrete is ready for the next stage of work, the main result should be considered not only as a percentage, but also as the converted strength in MPa. This approach is more useful when the result must be compared with design requirements or internal work criteria.
Normative reference. The concrete class is selected with reference to EN 206 Concrete. Specification, performance, production and conformity. For structural use and verification of loading conditions, EN 1992-1-1 Eurocode 2. Design of concrete structures is used. For execution of concrete works and timing of site operations, EN 13670 Execution of concrete structures should also be taken into account.
FAQs
Why does the calculator show both percentages and strength in MPa?
The percentage shows what share of the class strength the concrete has approximately reached at the specified time. Strength in MPa is needed for practical comparison with design requirements, test results, and internal criteria for construction work.
Why does temperature affect concrete strength gain so much?
Concrete curing is related to cement hydration, and its rate depends on temperature. At a higher average temperature, the reactions proceed faster, so the concrete strength calculator shows faster strength gain at early ages.
Why is 28 days used as the basis?
An age of 28 days is used as the standard reference point for comparing actual strength with the concrete class. For this reason, the online concrete strength gain calculation is structured so the result can be interpreted relative to this basic period.
Can this calculation be used to decide when formwork can be removed?
The calculator helps estimate the curing rate and compare different curing scenarios. However, for a decision of this type, not only the percentage but also the required member strength, loading scheme, structural type, and European execution rules are important.
What is the difference between cylinder strength and cube strength?
These are strength values determined on specimens of different shapes, so the numerical values for the same class are different. In the European concrete class designation, the cylinder strength is shown first, followed by the cube strength after the slash, for example C25/30.