A full analysis of the uncertainties in the calculation of the amount of gas absorbed or desorbed by a sample using the gravimetric technique has been made. The gravimetric technique provides a near direct measurement of the uptake of gas by continuously measuring the weight of the sample plus absorbed gas and is frequently used to characterise the uptake of hydrogen in potential hydrogen storage materials. These calculations provide the capability to determine the uncertainty in the final gas uptake measurement given knowledge of the measurement uncertainties. Uptake of hydrogen gas on activated carbon has been modelled to allow individual constants such as the buoyancy volumes as well as measurement uncertainties to be changed while determining the effect on the uptake uncertainty. The measurement uncertainties critical to minimising the overall uptake uncertainty have been identified and a comparison with another popular gas uptake method, the Sieverts technique, has been made. Crown Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.