Period Calculator
Period Calculator predicts your next several periods and fertile windows from your last period date, cycle length and typical period duration — shown on an easy-to-read calendar.
Last updated: July 2026
A period calculator predicts your upcoming periods and fertile windows from the first day of your last period and your average cycle length. It projects each cycle forward so you can plan ahead.
How to use Period Calculator
- Enter the first day of your last period.
- Set your average cycle length and how many days your period lasts.
- Choose how many cycles to predict and read the calendar and dates.
Frequently asked questions
How does it predict future periods?
It adds your cycle length repeatedly from your last period start date and marks each predicted period and fertile window.
What if my cycles are irregular?
Predictions are most accurate with regular cycles (21–35 days). For irregular cycles, treat the dates as rough estimates.
Is my data stored?
No — everything is calculated in your browser and nothing is saved or sent anywhere.
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