Concrete compressive strength calculator
Enter the maximum load for each specimen and get the compressive strength per EN 12390-3, to the nearest 0.1 MPa. For cubes and cylinders, no sign-up.
Maximum load at failure (kN)
Result
Enter a maximum load to see the strength.
For guidance only. A test report must follow the full procedure of EN 12390-3, including the specimen checks and the verified accuracy of the testing machine.
How compressive strength is calculated
EN 12390-3 defines the compressive strength as the maximum load at failure divided by the cross-sectional area of the specimen:
fc = F / Ac
fc - compressive strength (MPa) · F - maximum load at failure (N) · Ac - cross-sectional area (mm²)
For a cube the area is the side squared (150 mm gives 22,500 mm²); for a cylinder it is π × d² / 4 (150 mm gives 17,671 mm²). Because 1 N/mm² equals 1 MPa, a load in kN is multiplied by 1,000 before dividing. The standard asks for the result to the nearest 0.1 MPa.
Worked example
A 150 mm cube fails at 742.5 kN: 742,500 N ÷ 22,500 mm² = 33.0 MPa.
Questions engineers ask
What is the difference between cube and cylinder strength?
A cylinder (150 × 300 mm) gives a lower strength than a cube of the same concrete, so EN 206 names both: in C25/30, 25 MPa is the characteristic cylinder strength and 30 MPa the characteristic cube strength. Compare cube results with the second number and cylinder results with the first.
At what age are concrete cubes tested?
Most specifications call for 28 days, often with 7-day (and sometimes 2- or 3-day) breaks to follow early strength gain. The ages come from the project specification, not from EN 12390-3.
Does the calculator check the specimen dimensions?
No. It uses the size you enter. Within the tolerances of EN 12390-1 the nominal size may be used; otherwise enter the measured side or diameter.
Doing this for hundreds of cubes a month?
LabFlow calculates every specimen as it is entered, schedules the 7- and 28-day breaks when the cube is logged, and puts the results on an approved report your client downloads from their own portal.