The Ovulation Calculator estimates your most fertile days so you can plan around them. Enter the first day of your last period and your average cycle length, and it works out your likely ovulation date and fertile window — the days when conception is most likely — for the next few cycles.
It’s useful whether you’re trying to conceive or simply tracking your cycle. Everything runs in your browser, so none of the dates you enter are uploaded or stored.
How to find your fertile window
- Select the first day of your last period.
- Set your average cycle length — 28 days is typical.
- Read your estimated ovulation date and the fertile window around it.
- Use the next-3-cycles list to plan a few months ahead.
How ovulation and the fertile window are estimated
Ovulation usually happens about 14 days before your next period starts, so for a 28-day cycle it falls around day 14. The fertile window is the ovulation day plus the five days before it, because sperm can survive in the body for up to five days while the egg lives for about 24 hours.
Cycle length varies from person to person and month to month, so these are estimates. If your cycle is longer or shorter than 28 days, adjust the slider for a better prediction.
Tips for tracking fertility
- The two or three days leading up to ovulation are the most fertile.
- Signs of ovulation include a rise in basal body temperature and changes in cervical mucus.
- Ovulation predictor kits detect the hormone surge a day or so before ovulation.
- Irregular cycles make calendar predictions less reliable — a doctor can help if cycles vary a lot.
Frequently Asked Questions
When am I most fertile?
You are most fertile in the few days leading up to and including ovulation. This calculator highlights that window based on your cycle, typically the ovulation day plus the five days before it.
How accurate is an ovulation calculator?
It gives a good estimate for regular cycles, but ovulation timing naturally varies. For more precision, combine it with ovulation predictor kits or basal body temperature tracking.
Can I use this to avoid pregnancy?
Calendar-based methods are not a reliable form of contraception on their own, because ovulation can shift. Speak to a doctor about dependable contraception.
What if my cycle is irregular?
Irregular cycles make any calendar prediction less accurate. Use your average cycle length as a starting point, and consider tracking physical signs of ovulation as well.