Evaluating sealing capacity against the air leakage from unlined underground caverns for compressed air energy storage (CAES), a large-scale energy storage technology, is usually costly and time consuming. This paper presents an iterative method that can quickly estimate the air leakage rate of an unlined CAES cavern with adequate accuracy and requires fewer parameters than numerical simulations. The field tests of a pilot cavern in Japan and the NK1 cavern of the Huntorf plant as well as some numerical simulations were used as case studies to verify the proposed method. In these verifications, the proposed method achieved satisfactory results in terms of air leakage and cavern pressure. A sensitivity analysis was also conducted to examine the dependence of the air leakage from an unlined CAES cavern on the cavern characteristics and operating conditions. The most influential parameters were rock permeability, cavern radius, and the mass rate of injected air. Rock permeability should be smaller than 2.5 x 10(-19) m(2) to achieve a daily leakage percentage of less than 1% for a dry CAES cavern under pressure between 5 and 8 MPa. Moreover, a large cavern radius and a large mass rate of injected air could decrease a daily leakage percentage. (C) 2018 Elsevier Ltd. All rights reserved.