Free diving is a sport that in the past few years has increased in importance, considering the number and the level of practising athletes. In order to reach depths close to 100 m in constant ballast new physical and mental training techniques have been developed, but also different innovative materials and new shapes have been adopted for the fins. Usually the introduction of such improvements is based on manufacturer’s experience, while their evaluation is remitted to athlete’s opinions and not to quantitative measures. The aim of this study is to assess a new simple methodology to quantitatively determine the performances of free diving fins. This methodology will be shown in detail and the results of the analysis on a very popular fin will be reported.