Biohydrogen potential of biodegradable samples is often evaluated with batch experiments, where gas collection devices (gasbags, syringes, water displacement units) are used for gas quantification and analysis. When calculating biohydrogen production rate and yield, the biogas distribution in headspace and gas collection devices is often assumed to be identical. For the first time, the effect of gas generation rate and headspace volume on gas distribution in these two locations was examined. Based on abiotic studies, a model for the biogas distribution was developed and experimentally validated. The results suggested a minimum 5% and a maximum 30% difference in biogas concentration between headspace and gas collection unit when the volume of biogas produced was between 0.1 and 20 times the headspace volume. The maximum difference (ca. 30%) was detected when volume of biogas production reached 1.6 times the volume of headspace. While a precise difference was evident with abiotic experiments, such a predictable difference was not observed with biotic experiments due to the dynamic nature of biological systems. Given this difference of biogas measurements between headspace and gas collection device was largely ignored in past research, the purity of produced biohydrogen reported in some literature remains notably inaccurate, specifically when concentrations reported were derived only with a measurement of biogas in either headspace or gas collection device. Further, this error in measurement in batch experiments carried out by industry to examine biogas potential, gas quality assurance and quality control has also likely negatively impacted industry operations, industry insights, strategic decision making, and regulatory compliance. This error in biogas measurement in batch experiments can be rectified by considering the biogas composition in both headspace and gas collection device.