Ovulation and fertile days calculator
It shows the days when the chance of pregnancy is highest, for regular cycles from 21 to 45 days.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter first day of your last period and average cycle length.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Ovulation = first day of last period + cycle length - 14. Fertile window = 5 days before ovulation through ovulation day. Next period = last period + cycle length.
Example
Inputs: First day of your last period = 2026-09-01, Average cycle length = 28 days. Result: Expected ovulation day: Tuesday, September 15, 2026 (2026-09-15); Fertile window starts: Thursday, September 10, 2026 (2026-09-10); Fertile window ends: Tuesday, September 15, 2026 (2026-09-15).
Limits and caveats
- Ovulation is not always exactly 14 days before a period; it can shift by several days.
- Do not use this as birth control.
- With irregular cycles, ovulation tests or medical monitoring are more reliable.
Questions about this tool
Why is the fertile window six days?
Sperm can live up to 5 days and the egg about 1 day, so sex in the 5 days before ovulation or on the day itself can lead to pregnancy.
Which days are most fertile?
Studies show the two days before ovulation and ovulation day have the highest chance.
Can pregnancy happen outside these days?
Yes. Ovulation can move from month to month, so the calculated days are an estimate, not a guarantee.