When is your next period?
Enter the first day of your last period, your average cycle length, and how long your period typically lasts. We'll predict the next six.
Cycle math is right within a few days for people with regular cycles, less reliable if your cycles vary by more than a week. Stress, illness, sleep deprivation, weight changes, and hormonal birth control all shift cycle length.
The mean cycle length in a 600,000-cycle real-world dataset was 29.3 days, not the textbook 28 — and the spread is wide. Cycle length also shortens by roughly 0.18 days per year of age between 25 and 45 (Bull et al. 2019). The "28-day cycle" is a useful default, not a description of any specific person.
For higher accuracy, pair this with basal body temperature tracking or LH-strip ovulation tests. Apps like Clue and Flo use the same math but adjust over time as you log actual periods.
Sources: ACOG Committee Opinion 651, "Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign" (2015, reaffirmed 2021); Bull JR et al. npj Digital Medicine 2:83 (2019), "Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles."