When the ionic strength of a solution is too high for the Debye-Hückel limiting law to be valid (ie the molality of the solution is too high), the mean activity coefficient may be estimated from the extended Debye-Hückel law:

where B is a dimensionless constant, most simply regarded as a fudge factor

This expression allows reasonable estimation of the mean activity coefficient for molalities up to approximately 0.1 mol kg^{-1} . However for molalities of 1 mol kg^{-1} or more, agreement with the experimental value is once again very poor.