The most commonly used fan laws in simplified form are:

  Fan Law Formula
1 CFM varies Directly with RPM (CFM 1  / CFM 2 = RPM 1 / RPM 2)
2 SP varies with the SQUARE of the RPM (SP 1 / SP 2) = (RPM 1 / RPM 2)2
3 HP varies with the CUBE of the RPM (HP 1 / HP 2 = (RPM 1 / RPM 2)3