All springs relax under load. A compression spring will shorten slightly, which can be measured, but usually shows itself as a loss in load over time (assuming the spring is being held at a fixed length). The predictions in Spring Calculator Professional refer to this loss in load, measured as a percentage.



Here you can see a prediction that if the spring was held at position 2, it would lose 2.75% of its load after 1000 hours at 20 degrees. Changing the time or temperature will alter this prediction.


Note that the prediction is always rounded off to the nearest 0.25%, because it is inherently an estimation, interpolated for the stress, time and temperature values given. IST would always recommend testing to your exact parameters to achieve a more accurate prediction.