Ovulation Calculator

• Next period = first day of your last period + cycle length. For the second and third cycles, the cycle length is added two and three times.
• Estimated ovulation day = next period − 14 days
• Fertile window = from 5 days before the estimated ovulation day to 1 day after it, 7 days in all
• Irregular cycles: the fertile window runs from the first fertile day found with your shortest cycle to the last fertile day found with your longest cycle. The ovulation day and the next period are shown as ranges found with the two cycle lengths.
• Dates are added and subtracted on the calendar. Month ends, new years and February 29 in leap years are all counted as they fall.

Where these rules come from

• The American College of Obstetricians and Gynecologists (ACOG) explains that in a 28-day cycle ovulation happens about 14 days before the next period starts, and that pregnancy is possible from 5 days before ovulation to 1 day after it.
• A study published in the New England Journal of Medicine in 1995 (Wilcox et al.) found that almost all pregnancies began from intercourse in the six days ending on the day of ovulation. The fertile window here is those six days plus the day after ovulation.
• Korea’s National Health Insurance Service also describes ovulation as usually about 14 days before the expected next period.

If your last period started on September 20, 2026 and your cycle is a regular 28 days, you see:

With the same start date but an irregular cycle of 28 to 30 days, the first cycle looks like this:

• Fertile window: September 29 – October 7, 2026
• Estimated ovulation: October 4 – October 6, 2026
• Next period: October 18 – October 20, 2026
• The fertile window is 9 days, 2 days longer than with a regular cycle. It widens by the difference between your shortest and longest cycle.

• Calendar estimates work less well outside 26 to 32 days. If your average cycle is outside that range, or you choose Irregular, a note appears above the result. ACOG explains that the Standard Days method, one of the calendar-based methods, works best for people whose cycles are regular and between 26 and 32 days long.
• The time from ovulation to the next period varies from person to person. This calculator uses 14 days. The NHS says most people ovulate 10 to 16 days before their next period. A 2019 study of cycle-tracking app data (Bull et al.) found an average of 12.4 days, with most people between 7 and 17 days.
• Some dates may already be past. If you enter a period that started several months ago, some or all of the three cycles are before today. Those rows are marked Past dates. Enter the start of your most recent period to see dates ahead.
• New years and leap years are counted on the calendar. A period starting on December 20, 2026 with a 30-day cycle gives a fertile window of December 31, 2026 – January 6, 2027 and an estimated ovulation day of January 5, 2027. A period starting on February 10, 2028 with a 28-day cycle counts February 29, so the estimated ovulation day is February 24, 2028 and the next period is March 9, 2028.
• Values it does not accept: a date after today, a date more than one year ago, a cycle shorter than 20 or longer than 45 days, or a shortest cycle longer than the longest. The calculator shows the reason under the field instead of a result.
• The same value for shortest and longest gives the same result as a regular cycle.

What is the difference between the ovulation day and the fertile window?

The estimated ovulation day is the single day on which this calculation expects you to ovulate. The fertile window is the stretch around it when pregnancy is possible. Here it is 7 days, from 5 days before the estimated ovulation day to 1 day after it.

Can’t I just add 14 days to the start of my last period?

Only if your cycle is 28 days. Ovulation happens about 14 days before the next period, counting backwards, not 14 days after the last one. If your last period started on September 20, 2026 and your cycle is 35 days, your next period is expected on October 25, 2026, so the estimated ovulation day is October 11, 2026, not October 4. The fertile window is October 6 – October 12, 2026. The two methods are 7 days apart.

Why does another calculator end my fertile window a day later?

Calculators that use the rhythm method (the Ogino–Knaus formula) count the first day of your last period as day 1. They take the fertile window from day (shortest cycle − 18) to day (longest cycle − 11). That gives the same first day as this calculator and a last day one day later. In the irregular example above, this calculator ends the window on October 7, while the rhythm method ends it on October 8, 2026. Sources do not agree on the exact numbers to subtract, so this calculator uses the same basis for irregular cycles as for regular ones.

My cycle length changes every month. What should I enter?

Choose Irregular and enter your shortest and longest cycle from the last few months. The fertile window widens to cover the difference. The more your cycle varies, the less accurate any calendar estimate becomes.

Where does the information I enter go?

The dates you enter and the results are worked out only in your browser. They are not sent to our server, to visitor statistics or with ad requests.

By default nothing is saved on your device, so the dates are gone when you close the page. If you turn on Save on this device and calculate, your period start date and cycle are saved in this browser. Anyone else who uses this device can see them. Turn it off to delete what was saved.

If you share a result, your period start date and cycle are included in the link. Anyone who receives the link, and anyone in the chat where you send it, can see them.

Can I use this calculator for birth control?

No. This calculator is meant for people who are trying to conceive and want to know their likely fertile days ahead of time. Calendar methods are easy to get wrong, because the ovulation day can shift from one cycle to the next. According to ACOG, with typical use of methods like these, 12 to 24 out of 100 people become pregnant in the first year.

This result is not a medical diagnosis. If your cycles are irregular, or you are worried because you have been trying to get pregnant for a long time, talk to a doctor, such as a gynecologist.