About the Period Calculator
Knowing when your next period is due helps you plan travel, events and self-care. Enter your last period and usual cycle length to see your next period dates and fertile days.
How to use the Period Calculator
- Enter the first day of your last period.
- Enter your average cycle and period length.
- See your next three period dates.
- Update the date each month for the best accuracy.
Formula used
Next period start = last period start + cycle length (repeated for each cycle). Period end = start + period length − 1.
Frequently asked questions
What is a normal cycle length?
Cycles of 21–35 days are common in adults; teens may have longer or more irregular cycles.
Why is my period late?
Stress, travel, weight change, illness, thyroid problems, PCOS or pregnancy can delay a period. See a doctor if it is often irregular.
Is my data saved?
No. Everything is calculated on your device and nothing is stored or sent anywhere.
Last reviewed: November 7, 2026