Ovulation and Fertile Window Calculator
This ovulation calculator estimates when you may ovulate and which days might be most fertile based on the first day of your last menstrual period and your usual cycle length. Every cycle is different, so this tool provides an estimate only and cannot be used for birth control or medical diagnosis.
Estimate your ovulation day and fertile window
Enter the first day of your last menstrual period and your average cycle length. The calculator will estimate your next ovulation day and a possible fertile window.
How ovulation and the fertile window are estimated
In a typical cycle, ovulation often happens about 14 days before the next period. If your cycle is 28 days long, ovulation may be around day 14. If your cycle is longer or shorter, the likely ovulation day shifts accordingly.
- Ovulation day (estimated): about cycle length minus 14 days after LMP
- Fertile window (estimated): about 5 days before ovulation and 1 day after
Sperm can live in the reproductive tract for several days, so pregnancy is most likely if unprotected intercourse happens in the days leading up to ovulation and on the day of ovulation.
Because cycles can vary, tracking other signs (such as cervical mucus, ovulation predictor kits or basal body temperature) together with your provider’s advice can give a clearer picture of your fertile days.
Ovulation Calculator – Frequently Asked Questions
How accurate is an ovulation calculator?
Ovulation calculators are based on average cycle patterns, so they can only provide an estimate. Many people have cycles that change from month to month, and ovulation can be earlier or later than predicted. For more precise information, you can combine this with ovulation predictor kits and guidance from your healthcare provider.
Can I use this calculator to avoid pregnancy?
No. This calculator is not reliable enough to be used for birth control. If you want to avoid pregnancy, talk to your healthcare provider about contraceptive options that are right for you.
Does stress or illness affect ovulation?
Yes. Stress, illness, travel, changes in weight, certain medications and other factors can all affect when or whether ovulation happens in a given cycle. This is one reason why cycle-based estimates are not exact.
What if my menstrual cycles are irregular?
If your cycles vary a lot from month to month, predicting ovulation with a simple calculator is much less accurate. In that case, it is especially helpful to speak with your healthcare provider and consider additional tracking methods or tests.
When should I talk to a doctor about fertility?
In general, many guidelines suggest talking to a doctor if you have been trying to conceive for a year (or six months if you are 35 or older), or sooner if you have very irregular cycles, a history of pelvic infections, endometriosis or other health concerns.
