Calculator
Ovulation & Fertile Window Calculator
Estimate your fertile window, likely ovulation day and next period from the start of your last period and your usual cycle length.
Ovulation & Fertile Window Calculator
Enter the first day of your last period and your usual cycle length to see your estimated fertile window for the next three cycles.
Your estimate
- Fertile window
- Estimated ovulation
- Next period due
- Earliest pregnancy test
This is an estimate based on a regular cycle. Ovulation can shift from cycle to cycle with stress, illness and travel, and it cannot be used to prevent pregnancy. Ovulation predictor (LH) kits or a clinician can confirm timing.
About this tool
Pregnancy is possible only in a short window each cycle. An egg lives for roughly 12 to 24 hours after it is released, while sperm can survive in the reproductive tract for up to five days. That is why the fertile window is usually counted as the five days before ovulation plus the day of ovulation.
This calculator assumes ovulation happens about 14 days before your next period starts, which is typical, and lets you adjust the luteal phase if you know yours is shorter or longer. If your cycles are irregular, the estimate can be off by several days. Ovulation predictor kits, cervical mucus changes and basal body temperature can narrow it down.
Having intercourse every one to two days during the fertile window gives the best chance of conceiving. You do not need to time every encounter perfectly.
What to keep in mind
- Estimates assume a regular cycle. They cannot confirm that ovulation actually happened.
- This is not a method of contraception.
- If periods are irregular, very infrequent or absent, ask a clinician about testing rather than relying on a calculator.
References
We rely on government health agencies, professional medical organizations, peer-reviewed research and academic institutions. Read our sourcing standards.