Abstract: A biorhythm scale adapted to secure its operation and capable of directly observing the periodic day and the half-periodic day a physical, sensitive and intellectual rhythm for the individual or individuals in an accurate manner. The biorhythm calculator includes a guide interiorly of which a resilient non-slip element is firmly mounted not only to provide no slippage between vernier scales but also to eliminate an error in reading, and is comprised of a physical rhythm (P) scale, a sensitive rhythm (S) scale, an intellectual rhythm (I) scale, a day scale, and a month scale each of which is provided with racks at least one side thereof. Each of the vernier scales is slid over a fixed date scale indicative of the number of days in a month while in engagement of the racks with the non-slip element to thereby read out the biorhythm in the period as desired.