Ovulation Calculator
Estimate when you ovulate and your most fertile days for the next three cycles.
About this tool
Assumes ovulation about 14 days before your next period and a fertile window from five days before ovulation to one day after. Cycles vary from month to month, so treat the dates as estimates. It is not a form of contraception. Nothing you enter leaves your browser.
You can only get pregnant on a few days each cycle: the days leading up to ovulation and the day of ovulation itself. If you are trying to conceive, knowing roughly when those days fall helps. Enter the first day of your last period and your usual cycle length to see your estimated ovulation day, your fertile window and your next period, for this cycle and the next two. The dates you enter are not stored or sent anywhere.
How to use the Ovulation Calculator
- Enter the first day of your last period.
- Enter your usual cycle length, counted from the first day of one period to the day before the next; 28 days is average.
- Read your estimated ovulation day and fertile window.
- Check the following two cycles listed underneath.
A 30-day cycle starting 1 October
If your last period started on 1 October 2026 and your cycles are 30 days long, your next period is due on 31 October. Ovulation is usually about 14 days before that, so around 17 October, and the fertile window runs from 12 to 18 October. The next cycle would start on 31 October, with ovulation around 16 November.
Why the fertile window is six days
An egg survives for about 12 to 24 hours after ovulation, but sperm can survive in the body for up to five days. That makes the five days before ovulation and the day of ovulation the fertile window, with the two or three days just before ovulation the most likely to lead to pregnancy. The second half of the cycle, from ovulation to the next period, is fairly steady at about 12 to 16 days, so ovulation is counted back from the expected next period rather than forward from the last one. Ovulation tests, cervical mucus changes and a rise in basal body temperature can confirm it more precisely. If your cycles are irregular, or you have been trying for a year (six months if you are over 35), speak to a doctor.
Frequently asked questions
How accurate is an ovulation calculator?
It gives a good estimate for regular cycles, but ovulation can shift by several days. Ovulation predictor kits or temperature tracking are more precise.
Can I use this to avoid pregnancy?
No. Calendar estimates are not reliable enough for contraception. Speak to a healthcare provider about effective methods.
What if my cycle is irregular?
Use your average cycle length, but expect the dates to be less reliable. Tracking signs of ovulation helps more when cycles vary.
When should I take a pregnancy test?
From the first day of your missed period, about 14 days after ovulation, most home tests are reliable.