According to the UNE 100-230-95 standard, when a fan has been tested under certain conditions but we want to operate it in some different conditions, you can use equations called "Fan Laws". These laws will allow us to determine precisely the new performance of the fan from the tests carried out under normal conditions.
In addition, these laws allow to determine the performance of geometrically similar fans from the characteristics of the tested fan.
In this document you will find the summary of all the necessary equations to make the calculations of the equivalences between fans with geometric similarities.
See document with summary of calculations for geometrical similarities between fans: Fan laws