An analysis of the uncertainties in the calculation of the amount of gas absorbed or desorbed by a sample has been made for the manometric or Sieverts technique, commonly used for the determination of hydrogen uptake on potential hydrogen storage materials. The analytical derivations provide a means to calculate the uncertainty in each step of a hydrogen absorption isotherm. In addition, a Sieverts system is modelled to allow a detailed investigation of the effect of volumes and other parameters as well as the relative impact of the different measurement uncertainties. Reliable uncertainty estimates are especially important for authoritative measurements of gas uptake by carbons and other nanostructured materials used for hydrogen storage. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.