The standard ISO 8996 provides methods for the determination of metabolic rate from measured oxygen consumption ( M _VO2 ), as well as simplified estimation algorithms based on heart rate ( M _HR ). We quantified the accuracy of these methods by comparing M _HR with M _VO2 measured in 373 climatic chamber experiments under different workloads and widely varying heat stress conditions. While our results confirmed the 5% accuracy level for M _VO2 , M _HR considerably overestimated M _VO2 due to the rise in core temperature concomitantly increasing heart rate by approximately 30 bpm/°C resulting in an overall error of 43%. After individually correcting for this bias the accuracy was 10–15% as stipulated by the standard. Thus, methods correcting for the thermal component of heart rate, e.g. by introducing intermittent resting periods of sufficient length of at least five min when investigating heat stress at workplaces, should become a mandatory element in the ongoing revision of the relevant standards.