This paper presents an algorithm that evaluates the current state of an asset fleet containing sulphur hexafluoride (SF6) and estimates its leakage in future electrical power substation projects. The algorithm uses simple models and easy tools to facilitate decision making for transmission and distribution system operator companies. The algorithm is evaluated using data provided by ENEL-CODENSA. The corresponding results are shown, and the estimation values obtained are compared with leakage records in existing assets which helps to understand the advantages and limitations of the algorithm.