Recently automotive crashworthiness trend is the use of thin-walled composite impact attenuators in specific vehicle zones, ensuring the greater quantity of kinetic energy absorption. The paper aims at developing an analytical procedure in order to capture the energy absorption capability of impact attenuators with a square frusta geometry to adopt in the front of a car. An energetic approach is addressed taking into account the energy contributions responsible for the absorption during crushing. Comparison between analytical and numerical data, using an explicit dynamic code as LS-DYNA, shows the efficiency of the proposed relatively simple model for predicting energy absorption of axially collapsing composite shells