Period Calculator
For informational purposes only — not medical advice.
How to use this period calculator
This period calculator uses the dates you tap on the calendar to estimate your next period, ovulation day, and fertile window. It works best when you add the first day of several recent periods so the calculator can learn your typical cycle length.
- Tap the first day of each recent period on the calendar.
- Confirm your average period length if needed.
- Review your predicted next period date, ovulation day, and fertile days.
How the period calculator estimates your next period
- Next period start = last period start + average cycle length
- Next period end = next period start + average period length - 1
- Estimated ovulation day = next period start - 14
- Standard fertile window = ovulation - 5 to ovulation + 1
- Conservative fertile window = ovulation - 5 to ovulation + 4
- Rhythm method window uses your shortest and longest tracked cycles when you have enough history
The result is an estimate, not a guarantee. Real cycles can move earlier or later, even when your averages look stable.
For a deeper walkthrough, read How to Calculate Your Next Period.
How accurate is a period calculator?
A period calculator is most useful when your cycle is fairly regular and you enter several recent periods. If your cycle length changes a lot from month to month, the predicted next period date and ovulation day become less precise.
What can shift your next period date?
- Stress and sleep changes
- Travel and time zone changes
- Illness and medications
- Major changes in routine, diet, or exercise
- Hormonal birth control, postpartum changes, and perimenopause
Period calculator tips for irregular cycles
If your periods are irregular, track at least 3 to 6 completed cycles and treat every result as a range instead of an exact day. The more history you add, the more useful your averages become.
For irregular patterns, start with this irregular cycle guide.
Period Calculator FAQs
Frequently asked questions
Quick answers about period predictions, ovulation estimates, accuracy, and privacy.
From the blog
Start with one practical guide, then branch into related cycle and period tracking articles.
Fertility Period Calculator: What It Means and How to Use It
Learn what a fertility period calculator estimates, why fertile days are a window instead of one exact date, and how to use cycle history more realistically.
Curated to pair with the calculator and FAQ above.