This paper aims at finding a proper way to estimate sloshing severity. First, the concept of sloshing severity RAO (SSR) is introduced, and the wave elevation on the liquid free surface is chosen as an initial index for the rough prediction of sloshing severity. Then, compared with experimental data from a 3D regular model test, this index is adjusted and a new index is generated. One step further, sloshing severity under irregular sea states can be achieved by nonlinear combinations of the new index. For validation, the same model tank is tested under a set of irregular sea conditions, and peak pressures and impulse areas are taken as comparison standards. It is found that both numerical and experimental results show a similar tendency of sloshing severity. As a real ship application on the new index, the sloshing severity of a liquefied natural gas floating production storage and offloading (LNG-FPSO) is predicted under a low filling condition. Besides, the ship motion responses with and without sloshing effects are considered for the calculation of severity. From the present observation, this proposed methodology and generated new index is expected to be applicable to the selection of severe sea states for sloshing loads analysis.