Ovulation Calculator
Estimate your ovulation day and fertile window from your last period and cycle length.
This runs entirely in your browser. None of your dates are uploaded or stored anywhere.
How to use this tool
Enter the first day of your most recent period and your average menstrual cycle length. A typical cycle runs about 28 days, but anywhere from 21 to 35 days is common, so use your own number if you keep a menstrual cycle log. Press Calculate fertile window and this ovulation calendar fertility calculator returns your estimated ovulation day, your full fertile window, the peak conception days, when your next period is likely to start, and a suggested date to take a pregnancy test. You can copy or download the summary to keep with your records or share with your doctor.
How it works
Ovulation usually happens about 14 days before your next period starts, not 14 days after your last one. So this tool subtracts 14 from your cycle length to find how many days after your period ovulation is likely to occur. For a 28-day cycle that lands on day 14; for a 32-day cycle it lands on day 18. The fertile days span the five days before ovulation plus the day after, because sperm can survive in the body for several days while the egg lives for roughly one day. The two days right before and including ovulation are the peak conception window, when the chance of conceiving is highest.
A real example
Suppose your last period began on June 1, 2026, and your average cycle is 30 days. Ovulation is estimated at June 1 plus 16 days (30 minus 14), which is June 17, 2026. The fertile window runs from June 12 (five days before) through June 18 (one day after). The peak conception days are June 16 and June 17. Your next period would likely start around July 1, 2026, and a pregnancy test taken on or after that date would be the most reliable.
Common questions
How accurate is this ovulation calculator?
It gives a good estimate for people with regular cycles, but real ovulation can shift by a few days from month to month due to stress, illness, or hormonal changes. Tracking your cycle over several months, watching for physical signs, or using ovulation test strips will sharpen the estimate.
What are the fertile days?
The fertile days are the window when intercourse is most likely to lead to pregnancy. Because sperm can live up to five days and the egg lives about one day, the window spans the five days before ovulation through the day after. The two days leading up to ovulation are the peak conception window.
My cycle is irregular. Can I still use it?
You can, but the result will be less reliable. Use the average of your recent cycle lengths from your menstrual cycle log, and treat the dates as a rough guide. If your cycles vary widely, talk with a healthcare provider about other tracking methods.
When should I take a pregnancy test?
The suggested test date is the day your next period is expected. Testing earlier can give a false negative because hormone levels may still be too low. For the clearest result, wait until the first day of your missed period or a few days after.
Is this medical advice?
No. This calculator provides an educational estimate of your fertile window only. It is not a substitute for professional medical advice, contraception, or care. For family planning or fertility concerns, consult a qualified healthcare provider.