Period Calculator
Predict the dates of your next periods, your ovulation day and your fertile window from your last period and average cycle length.
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About the Period Calculator
This period calculator predicts your next menstrual periods from the first day of your last period and the average length of your cycle. It also estimates ovulation, which usually happens about 14 days before the next period, and your fertile window. Cycles can vary from month to month, so treat the dates as estimates.
How to use it
- Enter the first day of your last period.
- Enter your average cycle length and how many days your period usually lasts.
- Press Predict my periods to see the next six periods, ovulation and the fertile window.
How a menstrual cycle is counted
A cycle runs from the first day of one period to the day before the next. A typical cycle is 21 to 35 days, with 28 days as the average. Your cycle length is the number of days between the first days of two periods.
When to see a doctor
- Periods stop for three months or more and you are not pregnant.
- Cycles are shorter than 21 days or longer than 35 days.
- Bleeding is very heavy or lasts more than seven days.
- You have severe pain or sudden changes in your cycle.
Frequently asked questions
How do I calculate my next period?
Add your average cycle length to the first day of your last period. A 28-day cycle starting November 1 predicts the next period on November 29.
How accurate is a period calculator?
It is most accurate for regular cycles. Stress, illness, travel and hormones can shift your period.
When do I ovulate?
About 14 days before your next period. With a 28-day cycle that is around day 14.
What is a normal cycle length?
Between 21 and 35 days is normal for most adults.
Can I use this for birth control?
No. Calendar predictions are unreliable for preventing pregnancy.
Last reviewed: November 2026. Educational estimate only. Cycles vary and this is not birth control or medical advice.