Ovulation Calculator
Estimated ovulation day and fertile window from your last period and cycle length, shown on a calendar
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Enter the first day of your last period and your average cycle length to get an estimated ovulation day and fertile window by the calendar method, with the next six cycles in a table and on a calendar. It gives a rough idea of timing when planning a pregnancy.
How to use
- Enter the "First day of last period" and the "Average cycle length". Example values are filled in, so a result shows at once.
- Add a "Period length (optional)" to see the expected period days on the calendar.
- If your cycles vary, tick "My cycles are irregular (enter shortest and longest)" and enter the "Shortest cycle" and "Longest cycle". The fertile window widens.
- Read the "Calendar" (● period, ◆ fertile window, ★ estimated ovulation) and the "Next 6 cycles" table.
How it works
- Next period = period start + average cycle; estimated ovulation = next period − 14 days; fertile window = 5 days before to 1 day after the estimated ovulation day.
- The method assumes about 14 days from ovulation to the next period. Real ovulation differs between people and cycles, so this is not a method of contraception and not medical advice.
- No pregnancy chances or percentages are shown.
- Entries are kept in this browser only while "Save my entries on this device" is ticked, and "Clear saved entries" removes them.
Questions and answers
- How is the ovulation day calculated?
- Subtract 14 days from the next expected period. With a 28-day cycle that started on October 1, the next period is expected on October 29 and ovulation on October 15.
- When is my fertile window?
- This calculator shows the 5 days before the estimated ovulation day through 1 day after it. In the example above that is October 10 to October 16.
- How does it work with irregular cycles?
- The window runs from 5 days before the ovulation day of your shortest cycle to 1 day after that of your longest cycle. For cycles of 26 to 32 days that is cycle day 8 to day 20.
- When do I ovulate with a 35-day cycle?
- 35 − 14 = 21, so the estimate is 21 days after the period starts, which is cycle day 22.