The information related to the amount of vapour in the air, can be of critical importance for various processes, such as air-conditioning in buildings, drying or material processing. This information can be provided through different quantities such as the absolute humidity, relative humidity, dew-point temperature or wet-bulb temperature. Quite often, the user is more interested in a secondary quantity, which can be obtained through the use of appropriate relations or charts, rather than the directly measured one. The present work aims at proposing a methodology for the estimation of uncertainty related to the indirect humidity measurements. The analysis concentrates on the usual case which refers to the calculation of the values of derivative quantities, such as relative humidity or the amount of vapour in the air, through the direct measurement of quantities, such as dry-bulb temperature and dew-point temperature. The estimation of uncertainties is based on the propagation of probability distributions which could describe the available state-of-knowledge of the directly measured quantities, through the implementation of the Monte-Carlo simulation, which is consistent with the nonlinear characteristics of the hygrometric equations. (C) 2011 Elsevier B.V. All rights reserved.