Predict your next period in seconds
Enter the first day of your last period and your average cycle length. We'll map out your upcoming periods — and when to expect your PMS and fertile windows — right here in your browser.
Get reminded before it starts
MoodSwings predicts your period, PMS window, and fertile window automatically, learns your real cycle, and gives you a gentle heads-up — so you're never caught off guard.
How period prediction works
The simplest period prediction adds your average cycle length to the first day of your last period. If your last period started on the 1st and your cycle is 28 days, your next is estimated around the 29th. This calculator repeats that for several cycles and also marks your estimated PMS window (the few days before your period) and fertile window (around mid-cycle).
Real cycles vary, so a one-time calculation is a starting point. An app that learns from each cycle you log — like MoodSwings — adapts to your actual pattern, including irregular cycles, and gets more accurate over time.
Questions people ask
How does the period calculator work?
It adds your average cycle length to your last period start date to predict your next period, then repeats that for the following cycles. The more regular your cycle, the more accurate the estimate.
How do I know my cycle length?
Count from the first day of one period to the first day of the next. The average is around 28 days, but anywhere from 24 to 35 is common. If it varies, use a typical value and track a few cycles to refine it.
Why is my period prediction off?
Predictions are estimates. Stress, illness, travel, sleep, and natural variation can shift your cycle. Tracking several cycles in an app gives a far more personal and adaptive prediction than a one-time calculation.
Does MoodSwings predict my period automatically?
Yes — MoodSwings learns your cycle from what you log and predicts your next period, fertile window, and PMS window, updating as it goes. It is free to try on iPhone.