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."