Tritium-based mean residence time, tau(w), and flowrate, Q(w), of thermal waters from eleven different sites of Northern Thailand were used to evaluate the volume of reservoir waters contributing to the natural discharge, V-w, and the volume of the related geothermal reservoirs, Vres, assuming steady-state conditions and using simple relationships. Measured H-3 contents of the thermal waters of interest, apart from San Kamphaeng, range from 0.4 to 1.3 TU. Based on the well-mixed reservoir model, mean fluid residence times between 500 and 1700 years are obtained. For most thermal waters of Northern Thailand, calculated V-w range from 0.13 to 0.68 km(3), whereas V-res, vary from 13 to 68 km(3), assuming an effective porosity of 0.01. The only exception is San Kamphaeng, that has a high discharge leading to a calculated V-w of 2.2 km(3) and V-res of 220 km(3), although its mean fluid residence time of 955 years is within the range of other Northern Thailand thermal waters. Even though the uncertainties on both V-w and V-res are relatively high, the simple approach used here may be used to evaluate geothermal resources during the exploration Stage. The obtained V-res may be used in the volume method of geothermal resource assessment (which is usually utilized to evaluate the productive potential of unexploited and minimally exploited geothermal fields) and may be compared with the V-res estimated by means of other approaches. (C) 2015 Elsevier Ltd. All rights reserved.