In recent years, a series of pipeline leakage accidents frequently occur due to aging, corrosion and manmade damage. At present, most of related software focuses on the qualitative accidental assessment and localization of the leaking point while the study correlated with practical calculation of leakage volume is rare. An efficient quantitative calculation of the volume of leakage is essential to provide guidance for subsequent accidental management, environmental evaluation and division of the hazardous area. Connected with the alarm system and the online calculation module, a software with the function of online and offline calculation is developed on the basis of transient flow model coupled with hydraulic and thermal factors. A real case study is presented to demonstrate the relative error which is less than 5% and the results prove that it is of practical value and able to offer reference and guidance for engineers.