How it works
The calculator takes the first day of your last period, your average cycle length, and your average period length, and projects the next three expected start and end dates. It assumes your cycles are roughly regular — which they are for most people most of the time, but with normal ±2 day variation.
What “cycle length” means here
Day 1 is the first full day of bleeding. Cycle length is the count from day 1 of one period to day 1 of the next — a 28-day cycle means 28 days between starts, not 28 days between the *ends* of one period and the start of the next.
When to talk to a doctor
Sudden, persistent cycle changes — particularly missed periods that aren’t pregnancy, breakthrough bleeding between periods, or flow heavy enough to require pad changes more than once an hour — are worth flagging to your provider. Cycle changes are common during perimenopause, after starting or stopping hormonal contraception, postpartum, and during big life events; but they can also signal thyroid issues, polyps, or PCOS.