Avoiding leaks is essential for accurate measurement of biogas production by batch assays. Here we present a simple method for detecting leaks and correcting results, based on the change in bottle mass during incubation. Three experiments were carried out using pure chemicals, wastewater sludge, and other complex substrates to test and demonstrate the method, and leaks were detected in all three. The frequency and magnitude of leakage was related to headspace pressure and the number of times bottle septa had been punctured. Comparison to an independent estimate of leakage in two experiments showed that the proposed method is accurate. This mass-based approach can generally be used to detect leaks as small as 20% of total biogas or methane production, or lower when biogas production is high relative to the precision of mass measurements. Additional research is needed to improve the sensitivity of the method and to better understand the causes of leakage. Given the potential importance of leaks and the simplicity of leakage measurements, we recommend that this method is always used in batch biogas assays.