Indoor wet-bulb globe temperature (WBGT) has been recommended by many thermal environmental standards for evaluating the hot environment in naturally ventilated buildings. Indoor WBGT is equal to the weighted average of natural web bulb temperature and globe temperature, and thus it is not a function of indoor hot environmental parameters. It may be not possible to use indoor WBGT to determine acceptable hot environmental parameters during the design stage of naturally ventilated buildings. This study developed a simplified indoor WBGT formula based on the linearization of calculation equations of natural web bulb temperature and globe temperature, and then indoor hot environmental parameters, natural wet-bulb temperature, globe temperature and WBGT were measured in a typical naturally ventilated building to validate the proposed formula. The results show that indoor air temperature, mean radiant temperature, relative humidity and air velocity levels were 24 degrees C-40 degrees C, 30 degrees C-50 degrees C, 21%-75% and 0.1 m/s-3.0 mks. The relative errors between the measured and calculated indoor natural wet-bulb temperature, globe temperature and WBGT were all less than 5%. Hence, the proposed indoor WBGT formula was validated to be rational, which can be conveniently used to determine the acceptable hot environmental parameters during the design stage of naturally ventilated buildings. (C) 2019 Elsevier B.V. All rights reserved.