Period calculator
When your next periods are due, with your fertile days.
About this calculator
This period calculator predicts when your next period will start and end, based on your usual cycle. Enter the first day of your last period, your cycle length and how long your period lasts.
You'll also see your fertile window and estimated ovulation, and the dates of your next six periods.
How it works
Adds your cycle length to the first day of your last period to find the next start, and counts your period length from there. Ovulation is estimated at 14 days before each period.
Last reviewed:
Common questions
What is a normal cycle length?
Anywhere from about 21 to 35 days is common in adults, with 28 days as the average. Teenagers often have longer, less regular cycles.
Why is my period late?
Stress, illness, travel, weight changes, some medicines and pregnancy can all delay a period. If you might be pregnant, take a test.
When should I see a doctor about my periods?
If they stop for three months or more, come more often than every 21 days, last longer than seven days, or are very heavy or painful.
Sources and further reading
The story behind it
Where the idea came from: The word menstruation comes from the Latin mensis, month, because cycles last about a month.
People have tracked cycles against the moon and the calendar for thousands of years. Today period tracking is one of the most popular uses of health apps. This calculator does the same sum without storing anything: your dates stay in your browser.
Why you can trust this calculator
- It runs in your browser. Nothing you type into it is sent anywhere.
- The method is explained on this page, so you can check it yourself.
- Free to use, with no sign-up.
- Results are estimates, not medical advice.
- Spotted a mistake? Tell us and we'll look into it.
After 15 years as a mathematics educator, I built CalculStore to give students, teachers, professionals and anyone working with numbers simple, reliable tools.
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