In radionuclide metrology, the assessment of activity concentrations is directly related to the mass determination in quantitative source preparation. As far as primary measurements are concerned, the associated weighing uncertainty has a non-negligible contribution in the uncertainty budget. Several parameters influencing the mass determination have to be considered in source preparation and for the evaluation of the weighing uncertainty: technical specifications of calibrated balances, environmental quantities such as air buoyancy and other disturbance sources like electrostatic effects. The accuracy of mass determination is also conditioned by good practices in source preparation. This article reviews the different parameters in the evaluation of the weighing uncertainty. Its aim is also to give some guidance on how to achieve the smallest uncertainty.