Period Predictor Calculator

About this calculator

Menstrual cycle prediction works from two inputs: the start date of your last period and your average cycle length (the days from the start of one period to the start of the next, typically 21–35 days). Adding the cycle length repeatedly to the last start date projects the next several period start dates, while ovulation is generally estimated to occur about 14 days before the next period begins, in the luteal-phase model used by most cycle-tracking tools.

This calculator takes your last period start date and average cycle length and projects your next three period dates along with an estimated ovulation date. People tracking their menstrual cycle for general awareness, fertility-awareness planning, or simply to anticipate PMS and pack accordingly use it, keeping in mind that cycles vary naturally and this is a projection based on averages, not a guarantee — especially for anyone with irregular cycles.