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What this calculator does
Estimate a likely ovulation window from a cycle start date.
How to use this calculator
- Enter the first day of the period and cycle length if available.
- Review the calculation assumptions before using the result.
- The result identifies likely fertile dates.
How to calculate
Enter the values requested by the ovulation calculator.
Worked example
With the example values shown above, the ovulation day is 2026-01-15, and the fertile window is 2026-01-08 to 2026-01-16, and the estimated due date is 2026-10-08.
Understanding your result
The result identifies likely fertile dates.
- Cycles vary, so do not rely on this alone for contraception.
- Double-check values and units before calculating.
This calculator is educational and is not financial or medical advice.
Frequently asked questions
What does this ovulation calculator do?
Estimate a likely ovulation window from a cycle start date.
How do I use this ovulation calculator?
Enter the first day of the period and cycle length if available.
What does the result mean?
The result identifies likely fertile dates.
What should I check before relying on it?
Cycles vary, so do not rely on this alone for contraception.
Why is the fertile window shown as several days, not just one?
Sperm can survive for several days and the egg is viable for about a day after ovulation, so the fertile window spans the days leading up to and just after the estimated ovulation date, not ovulation day alone.
Does this work for irregular cycles?
It's less reliable for irregular cycles, since the calculation assumes ovulation occurs a fixed number of days before the next expected period based on your typical cycle length.