Ovulation Calculator
Free ovulation calculator: enter the first day of your last period and cycle length to find your fertile window, ovulation day and next period date.
About this tool
Ovulation usually happens about 14 days before your next period, and the fertile window covers the five days before it plus ovulation day itself.
Enter the first day of your last period and your average cycle length, and this ovulation calculator estimates your fertile window, ovulation day and next period.
How to use
Pick the first day of your most recent period on the calendar and type your usual cycle length (28 is average; anywhere from 21 to 45 works).
Press Calculate Fertile Window to see your estimated ovulation day, the six-day fertile window around it, and when your next period is expected.
The math behind it
Ovulation day = last period start + (cycle length - 14). Fertile window = ovulation day minus 5 days through ovulation day. Next period = last period start + cycle length.
These are estimates from averages - real cycles vary, so treat the window as a guide rather than a guarantee.
Frequently asked questions
How accurate is this ovulation calculator?
What is the fertile window?
My cycles are irregular. Can I still use this?
Related tools
💬 Comments
Have a question about this page, or an idea to make it better? Share it below — your feedback helps everyone who uses this site.