Human metabolism is a fundamental and important aspect of thermal comfort prediction. However, to date, most previous thermal comfort studies tended to treat metabolic rate as a constant value that only depends on activity level without consideration of ambient thermal conditions. In this paper, we explore the impacts of indoor thermal conditions on occupants' metabolic rate and try to describe such impacts in a straightforward way. Climate chamber experiments, including both physiological measurements and subjective questionnaires, were conducted at temperatures ranging from cold to hot with two clothing insulation levels. The results indicate that human metabolic rate can be significantly affected by thermal conditions such as ambient temperature and clothing insulation. The changing trend of metabolic rate can be expressed as a second-order polynomial equation with actual thermal sensations or other predicted values. Generally, a neutral thermal sensation corresponds to the lowest metabolic value, while the metabolic rate increases when thermal sensation departs from neutral to the cold or warm side. For instance, the metabolic rate at a sensation of 'cold' is 16.6% higher than it is in 'neutral' sensation. These findings, especially the modified equations describing metabolic rate changes, can serve as useful reference for future thermal comfort studies. (C) 2016 Elsevier B.V. All rights reserved.