When is my next ovulation?
Enter the first day of your last period and your usual cycle length. See your likely ovulation day, your six most fertile days and when your next period is due.
- Likely ovulation day
- Tuesday, October 6, 2026
Your likely ovulation day is Tuesday, October 6, 2026, and your fertile window is October 1, 2026 to October 6, 2026.
- Cycle starts
- September 22, 2026
- Fertile window from
- October 1, 2026
- Fertile window to
- October 6, 2026
- Ovulation on cycle day
- 15
- Next period starts
- October 20, 2026
- Due date if you conceive this cycle
- June 29, 2027
Answer for the example date Tuesday, September 29, 2026. It changes to today's date when the page loads.
Likely ovulation day: Tuesday, October 6, 2026. Your likely ovulation day is Tuesday, October 6, 2026, and your fertile window is October 1, 2026 to October 6, 2026.
When are your fertile days?
This calculator gives general information only. It is not medical advice and does not replace a doctor or other health care provider. Talk with a professional about your results. Terms of use
How to calculate
Estimates the ovulation day and the six-day fertile window from the first day of the last period and the cycle length: ovulation is about 14 days before the next period.
Example with the default inputs (First day of your last period September 22, 2026, Cycle length (days) 28, Luteal phase (days) 14, Today’s date September 29, 2026) on the example date Tuesday, September 29, 2026: Your likely ovulation day is Tuesday, October 6, 2026, and your fertile window is October 1, 2026 to October 6, 2026.
Method: Ovulation = cycle start + cycle length − luteal phase (14 days), where the cycle start is the last period moved on by whole cycles until ovulation is on or after today; the fertile window runs from 5 days before ovulation to the ovulation day.
- Ovulation happens a luteal phase (14 days unless you change it) before the next period starts.
- The fertile window is the 6 days ending on ovulation day: sperm can live up to about 5 days, the egg about a day.
- Cycles are taken as regular. Real ovulation can move by several days from cycle to cycle.
- If this cycle’s ovulation day has passed, the dates are for the next cycle with the same length.
- This is an estimate, not medical advice, and not a way to prevent pregnancy.
Worked examples
Each example is checked against the calculator on every build.
- First day of your last period 2026-09-01, Cycle length (days) 28, Luteal phase (days) 14 gives Likely ovulation day 2026-09-15, Fertile window from 2026-09-10, Fertile window to 2026-09-15, Ovulation on cycle day 15, Next period starts 2026-09-29, Due date if you conceive this cycle 2027-06-08.Source: Wilcox et al. 1995 (fertile window); ovulation 14 days before the next period; hand calculation in content.mdx
- First day of your last period 2026-02-10, Cycle length (days) 32, Luteal phase (days) 14 gives Likely ovulation day 2026-02-28, Fertile window from 2026-02-23, Ovulation on cycle day 19, Next period starts 2026-03-14.Source: hand calculation in content.mdx
- First day of your last period 2028-02-20, Cycle length (days) 26, Luteal phase (days) 12 gives Likely ovulation day 2028-03-05, Fertile window from 2028-02-29, Next period starts 2028-03-17, Due date if you conceive this cycle 2028-11-26.Source: Bull et al. 2019 (luteal phases vary, mean 12.4 days); hand calculation in content.mdx
- First day of your last period 2026-06-01, Cycle length (days) 28, Luteal phase (days) 14, Today’s date 2026-09-28 gives Cycle starts 2026-09-21, Likely ovulation day 2026-10-05, Fertile window from 2026-09-30, Ovulation on cycle day 15, Next period starts 2026-10-19, Due date if you conceive this cycle 2027-06-28.Source: hand calculation in content.mdx: first ovulation June 15; 105 days to September 28; ⌈105 ÷ 28⌉ = 4 cycles
- First day of your last period 2026-09-10, Cycle length (days) 30, Luteal phase (days) 14, Today’s date 2026-09-28 gives Cycle starts 2026-10-10, Likely ovulation day 2026-10-26, Next period starts 2026-11-09.Source: hand calculation in content.mdx: ovulation September 26 is 2 days before today; ⌈2 ÷ 30⌉ = 1 cycle
How it works
d is the first day of your last period, c the cycle length, L the luteal phase, and t today's date (you can change it under "Count from another date").
- Cycle start = d + k × c, where k is the smallest whole number 0, 1, 2, … for which the ovulation day below is on or after t: k = the larger of 0 and ⌈(t − (d + c − L)) ÷ c⌉. If this cycle's ovulation day is today or later, k = 0 and the cycle start is your last period. If it has passed, the dates are for the next cycle (or a later one, if your last period was longer ago), with the same length. If today's date is left empty, k = 0.
- Next period = cycle start + cycle length.
- Ovulation = next period − luteal phase, which is the cycle start + (c − L) days. The luteal phase is 14 days unless you change it (10 to 16 days).
- Ovulation on cycle day = cycle length − luteal phase + 1, counting the cycle start as day 1. It is the cycle day of ovulation, not today's cycle day.
- Fertile window = from 5 days before ovulation to the ovulation day, 6 days in all (Wilcox and others, 1995).
- Due date if you conceive this cycle = ovulation + 266 days. This is the same as ACOG's 280 days from a last period with ovulation on day 14.
Cycle length can be 21 to 45 days. All dates are calendar dates with no time of day.
The timeline shows your period, the fertile window, ovulation and your next period.
Assumptions
- Your cycles are regular, and this cycle is as long as you entered.
- Ovulation happens a fixed luteal phase before the next period. Real luteal phases vary; the average in a large study was 12.4 days (Bull and others, 2019).
- This calculator gives an estimate. It is not medical advice and not a method of birth control. Talk to a health care provider about fertility or birth control.
Worked examples by hand
The last three examples leave today's date empty, so k = 0.
Last period June 1, 2026, 28-day cycle, today September 28, 2026. This cycle's ovulation would be June 1 + 14 = June 15, which is 105 days before today. ⌈105 ÷ 28⌉ = 4, so the cycle start is June 1 + 4 × 28 = September 21. Ovulation = September 21 + 14 = October 5, day 15. Fertile window from September 30. Next period October 19. Due date if you conceive = June 28, 2027.
Last period September 10, 2026, 30-day cycle, today September 28, 2026. Ovulation would be September 10 + 16 = September 26, 2 days before today. ⌈2 ÷ 30⌉ = 1, so the cycle start is October 10, ovulation October 26 and the next period November 9.
Last period September 1, 2026, 28-day cycle, 14-day luteal phase. Next period = September 1 + 28 = September 29. Ovulation = September 1 + (28 − 14) = September 15, day 15 of the cycle. Fertile window = September 10 to September 15. Due date if you conceive = September 15 + 266 days = June 8, 2027.
Last period February 10, 2026, 32-day cycle. Next period = February 10 + 32 = March 14 (February 2026 has 28 days). Ovulation = February 10 + 18 = February 28, day 19. Fertile window from February 23.
Last period February 20, 2028, 26-day cycle, 12-day luteal phase. Ovulation = February 20 + (26 − 12) = March 5, 2028 (2028 is a leap year, so February has 29 days). The fertile window starts 5 days earlier, on February 29. Next period = February 20 + 26 = March 17. Due date if you conceive = March 5, 2028 + 266 = November 26, 2028.
Other questions people ask
When do I ovulate?
Ovulation usually happens about 14 days before your next period starts. In a 28-day cycle that is day 15, counting the first day of your period as day 1. In a 32-day cycle it is about day 19. The calculator counts back from your next period.
What is the fertile window?
It is the 6 days ending on the day of ovulation. In a study of 221 women trying to conceive, pregnancies came only from sex in the 5 days before ovulation or on the day of ovulation (Wilcox and others, 1995). Sperm can live in the body for up to about 5 days, and an egg lives for less than a day.
When is the chance of pregnancy highest?
The chance rises through the fertile window. In the 1995 study it went from about 10% for sex 5 days before ovulation to about 33% for sex on the day of ovulation. The calculator shows the whole window, because your exact ovulation day is uncertain.
How accurate is an ovulation calculator?
It is a guide. Ovulation moves from cycle to cycle, even in regular cycles. A study of more than 600,000 cycles found an average luteal phase (ovulation to next period) of 12.4 days, not 14. If you have tracked your own, enter it under More options. Ovulation tests or basal body temperature can pin down your own day.
Can I use this to avoid pregnancy?
No. Calendar estimates are not reliable enough to prevent pregnancy, because the fertile window moves from cycle to cycle. Talk to a health care provider about birth control.
What if my cycles are irregular?
Use your average cycle length, but the dates will be less certain. Very irregular cycles are worth discussing with a health care provider.