Since the early years of the past century, many scholars have focused their efforts towards designing models to better understand the way listeners perceive musical tension. From the existing models, Lerdahl's has shown strong correlations against tension judgements provided by human listeners and has been used to make accurate predictions of musical tension. However, a full automation of Lerdahl's model of tension has not yet been made available. This paper presents a computational approach to automatically calculate musical tension according to Lerdahl's model, with a publicly available implementation.