Abstract: A dial calculator for determining periods of fertility and sterility in a menstrual cycle of a human female. In one embodiment, the calculator includes first and second discs each having an annular display of first indicia representing thirty-one days of a calendar month. The second disc is smaller than the first disc and is adapted to be rotatably set in coaxial relation with the first disc so that the indicia of the second disc follow consecutively any one of the indicia on the first disc. A third disc is adapted to be rotatably set in coaxial relation with the second disc in respect of any one of the indicia on the first disc. The third disc is smaller than the second disc and includes an annular display of second indicia representing at least the days of a normal menstrual cycle including the periods of fertility and sterility and relates such days to the consecutively arranged indicia of the first and second discs. The three discs are simple to operate and display the indicia in improved form.