The invention concerns a system and a method for calculating the physical activity level (PAL) as an objective measure of the physical activity of an object over a certain period of time. The system (1) comprises an activity monitor (2) having three orthogonally mounted accelerometers (4, 5 and 6) for measuring the acceleration of the object, data processing means (7) and memory means (8). The activity monitor (2) further comprises communication means (12) for communicating with calculating means (3) for calculating the PAL. According to the claimed method the calculating means use the following algorithm: PAL=factor 1+factor 2*(variable 1)factor 3+factor 4*(variable 2)factor 5+ . . . +factor(2n)*(variable n)factor (2n+1). Herein are variable 1 through variable n related to the object and/or the system and are factor 1 through factor 2n+1 to be determined by validation of the system.