Internal floating-roof tank (IFRT) is widely used to store light oil products, but there exist oil evaporation from the rim gap of the floating deck and oil vapor diffusion into the atmosphere from several vents on the tank wall or tank roof. Based on the single-film mass transfer theory and the wind tunnel experiments, the equivalent film thickness (delta(E)) and the relevant evaporation rate model was proposed and numerically verified. The airflow speed, concentration and the evaporation loss rate in/from the tank were calculated using the model, and the influence of vent locations, the rim gap locations and the seal tightness of the annular rim gap on the loss rate was analyzed. The results show that when the floating deck elevates, the ambient wind speeds up, or the change of the vent locations from the roof to the wall, delta(E) decreases; the relationship of delta(E) with the oil type or the ambient temperature is not obvious; the effect of the seal tightness to the entire/partial rim gap on the loss rates is obvious. The results further reveal the interior mechanism of the vapor-air mass transfer and can provide important theoretical support for the design, management and improvement of IFRT. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.