How does period prediction work?
Cycle length, period duration and the formula behind it
The female cycle begins on the first day of the period and ends the day before the next period. The cycle length describes the number of days from one period start to the next. With a regular cycle, this reliably predicts the next period start: next period = last period + cycle length. For a 28-day cycle starting on January 1, the next period is therefore due on January 29.
The period duration (menstrual duration) indicates how many days the bleeding lasts – usually 3 to 7 days. Based on the period start and duration, our calculator determines the expected end of the next period as well as the period after next for better planning.
Important: this calculator is purely for cycle and period planning. It deliberately does not calculate fertile days or the ovulation date. If you want to calculate fertile days for family planning or other reasons, you can find this in our Ovulation Calculator.