Ovulation Calculator
Estimate your ovulation day, fertile window, and next period start date from your last period and cycle length.
This is an estimate based on average cycle timing. It is not a substitute for advice from a doctor or fertility specialist, and should not be relied on as a form of contraception.
How to Estimate Your Ovulation Day
- Enter the first day of your last period.
- Enter your average cycle length in days (typically 21-35 days; 28 is the default).
- Your estimated ovulation day, fertile window, and next period start date appear automatically.
- Adjust the cycle length any time to see how it shifts your estimated dates.
Frequently Asked Questions
How is ovulation day calculated?
Ovulation is estimated to occur 14 days before your next period starts. The calculator adds your cycle length to the first day of your last period to estimate your next period, then subtracts 14 days to estimate ovulation day.
What is the fertile window?
The fertile window is the estimated 6-day span with the highest chance of pregnancy — the 5 days before ovulation through ovulation day itself — since sperm can survive in the body for several days and the egg is viable for about 24 hours after release.
What if my cycle length varies from month to month?
Enter your average cycle length for the best estimate. If your cycles vary a lot, ovulation timing will vary too, and this calculator's estimate will be less precise — tracking basal body temperature or ovulation test strips over a few cycles can help narrow it down.
Does this work for cycles outside the typical 21-35 day range?
The calculator will still compute a result, but the underlying assumption (ovulation 14 days before the next period) is best supported by research for cycles in the roughly 21-35 day range — very irregular cycles are harder to predict with this method.
Is this a reliable form of birth control or a substitute for medical advice?
No — this is an estimate based on average cycle timing, not a guarantee, and individual ovulation varies. It is not a substitute for advice from a doctor or fertility specialist, and should not be relied on as a form of contraception.