We have developed a portable method to measure sweat rate (SR) under heat stress during field tests. We randomly divided 15 males and 17 females (23-78 yr) into a group, equation group (EG) to determine an equation to convert a unit of SR (mmHg) by the portable method to that (mg·min-1·cm-2) by the ventilation method, and another group, validation group (VG) to validate the equation. Since we repeated measurements twice in three subjects, we randomly assigned the two measurements to one of the two groups and analyzed the results in 18 and 17 subjects for EG and VG, respectively. Subjects cycled for 20 min at moderate intensity in a warm environment while chest SR was simultaneously measured with a capsule installed with 4.8 g of silica gel and two microfans (8.4 cm3 volume) and with another capsule (12.6 cm2 area) ventilated with dry air at 1.5 L·min-1. Since the esophageal temperature (Tes) threshold for increasing SR and the slope of SR at a given increase in Tes by the portable method (x) were in high agreement with those values obtained by the ventilation method (y) in both groups (all r > 0.88, P < 0.001), we determined regression equations for all subjects after pooling data from both groups: y = 1.11x - 3.99 and y = 1.05x + 0.01 when the 95% prediction limits were ±0.12°C and ±0.43 mg·min-1·cm-2·°C-1 with minimum mean differences over the range of 36.2°C-37.2°C and 0.2-2.4 mg·min-1·cm-2·°C-1, respectively, using Bland-Altman analysis. Based on these findings, we consider the portable device to be reliable enough to evaluate individual sweating capacity during field tests.NEW & NOTEWORTHY We developed a portable device to measure sweat rate continuously under heat stress during field tests, with precision similar to that obtained by the ventilation method, which has been used to evaluate individual sweat rate responses in laboratory tests. This new, portable device will provide more opportunities to determine factors influencing sweat rate in larger populations of subjects during field tests.