This work presents a gravimetric method for measuring biogas or methane production from anaerobic reactors, based on measurement of reactor mass loss. Results are most sensitive to error in biogas methane content, and less so to temperature and pressure. To evaluate the method, we applied it and volumetric methods to 133 laboratory-scale batch and semi-continuous reactors, ranging in size from 37 g to 8.0 kg of reacting mass. For most observations, the relative difference between the two methods was <10% when the “true” biogas composition was used in calculations. Small systematic differences observed in some cases were probably due to error in estimates of biogas pressure, temperature, and composition, as well as biogas leakage. Based on theory and observation, it is reasonable to expect relative accuracy better than 15% of the true value.