This paper addresses the international humidity community's need for standardization of methods for propagation of uncertainty associated with humidity generators and for handling uncertainty associated with the reference water vapour-pressure and enhancement-factor equations. The paper outlines uncertainty calculations for the mixing ratio, dew-point temperature and relative humidity output from humidity generators, and in particular considers controlling equations for a theoretical hybrid humidity generator combining single-pressure (1-P), two-pressure (2-P) and two-flow (2-F) principles. Also considered is the case where the humidity generator is used as a stable source with traceability derived from a reference hygrometer, i.e. a dew-point meter, a relative humidity meter or a wet-bulb psychrometer. Most humidity generators in use at national metrology institutes can be considered to be special cases of those considered here and sensitivity coefficients for particular types may be extracted. The ability to account for correlations between input variables and between different instances of the evaluation of the reference equations is discussed. The uncertainty calculation examples presented here are representative of most humidity calculations.