A complete description of the combined uncertainty for experimental measurements contains effects from multiple sources. This paper presents an analysis that uses density measurements as examples for which the sources are the measurement uncertainties associated with density, pressure, temperature, composition, gas constant, component masses and molar masses. Applying the analysis to two gas mixtures reveals the largest sources of error as the apparatus effect followed by the pressure effect. All other effects are minor by comparison in state-of-the-art measurements, but they are easy to include. It is also possible to determine how the accuracy of the balance used in a gravimetric preparation of the mixture affects the density uncertainty employing this technique. (c) 2012 Elsevier Ltd. All rights reserved.