A high-precision nitrogen boil-off method has been developed for measuring AC losses calorimetrically in HTS coils. A sensitivity of about 0.05 W was achieved through control of convection in a sample vessel and the use of a cryostat. In the case of a simple LN₂ reservoir instead of the cryostat, convection control in the sample vessel made it possible to measure losses with a sensitivity of about 0.5 W. AC loss measurements of Bi-2223 and Y-123 coils were examined using the cryostat and the simple reservoir, respectively. The results show that the reliability of the measurement is the same as for electrical measurement, and that the measuring procedure is simpler than any other method.