Ovulation and Fertile Window Calculator
Estimates your ovulation day and fertile window from the first day of your last period, your cycle length and your luteal phase.
Recent
Formula
How the calculation works
Enter the first day of your last period in the date field, which is day one of bleeding and not the day the period ended.
Enter your average cycle length in days, counted from the first day of one period to the first day of the next, and your luteal phase, which is normally about 14 days.
The tool subtracts the luteal phase from the cycle length to place ovulation, then marks the fertile window as five days either side of that date.
Common mistakes
- Using the last day of your period instead of the first day, which shifts every estimated date forward by the length of your bleeding.
- Assuming a 28-day cycle for everyone, when real cycles range from about 21 to 35 days and a longer cycle moves ovulation much later.
When to use it
- Use it to plan the days when conception is most likely if you are trying to get pregnant and want a simple window to focus on.
- Use it to understand your cycle better, since seeing the fertile window helps you time intercourse or a pregnancy test after a missed period.
Worked example
If your last period began on 1 January 2026, your cycle is 28 days and your luteal phase is 14 days, ovulation falls on 15 January 2026 and the fertile window runs from 10 January to 20 January 2026.
Common questions
How accurate is this ovulation estimate?
It is a calendar estimate that assumes a regular cycle, so if your cycles vary or you want confirmation, combine it with ovulation test strips or a basal body temperature chart.
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