Ovulation Calculator
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Results
Estimated ovulation Oct 15, 2026; fertile window Oct 10–15, 2026; next period Oct 29, 2026.
| Cycle | Period starts | Fertile window | Ovulation | Next period |
|---|---|---|---|---|
| 1 | Oct 1, 2026 | Oct 10–15, 2026 | Oct 15, 2026 | Oct 29, 2026 |
| 2 | Oct 29, 2026 | Nov 7–12, 2026 | Nov 12, 2026 | Nov 26, 2026 |
| 3 | Nov 26, 2026 | Dec 5–10, 2026 | Dec 10, 2026 | Dec 24, 2026 |
| 4 | Dec 24, 2026 | Jan 2–7, 2027 | Jan 7, 2027 | Jan 21, 2027 |
| 5 | Jan 21, 2027 | Jan 30 – Feb 4, 2027 | Feb 4, 2027 | Feb 18, 2027 |
| 6 | Feb 18, 2027 | Feb 27 – Mar 4, 2027 | Mar 4, 2027 | Mar 18, 2027 |
Do not use these dates as contraception. Ovulation can come earlier or later than predicted, even in regular cycles, and sperm can survive in the body for several days.
Ovulation varies from cycle to cycle, even in regular cycles. Ovulation (LH) tests and tracking cervical mucus or body temperature can help you see when it happens in your own cycle.
These dates are estimates from an average cycle, not medical advice. Talk to your doctor or midwife about your own cycle, fertility or pregnancy.
How to use the ovulation calculator
- Enter the first day of your most recent period: the first day of real bleeding, not spotting.
- Enter your usual cycle length, from 21 to 45 days. If it varies, use your average and expect the dates to shift.
- Leave the luteal phase at 14 days unless ovulation tests or temperature charts have shown yours is different.
- Read your fertile window and ovulation date, and the table for the next six cycles.
Ovulation formula
fertile window = ovulation − 5 days to ovulation day
next period = first day of period + cycle length
The time from ovulation to the next period, the luteal phase, is fairly constant for each person, and is usually about two weeks; the NHS says ovulation happens about 12 to 16 days before the next period. So the calculator counts back from the next expected period, which is why longer cycles give later ovulation. The fertile window comes from a 1995 study by Allen Wilcox and colleagues, who found that pregnancies came only from sex in the six days ending on the day of ovulation: sperm can survive for several days, while an egg lasts about a day.
Worked example
Your period started on Oct 1, 2026 and your cycle is 28 days. The next period is due on Oct 29, 2026, so ovulation is estimated 14 days earlier, on Thursday, October 15, 2026 (day 15 of the cycle), and the fertile window is Oct 10–15, 2026. With a 32-day cycle, ovulation moves to Oct 19, 2026; with a 28-day cycle but a 12-day luteal phase, it moves to Oct 17, 2026.
Ovulation day by cycle length
Cycle days counted from day 1, the first day of your period, assuming a 14-day luteal phase.
| Cycle length | Estimated ovulation | Fertile window |
|---|---|---|
| 21 days | Day 8 | Days 3 to 8 |
| 24 days | Day 11 | Days 6 to 11 |
| 26 days | Day 13 | Days 8 to 13 |
| 28 days | Day 15 | Days 10 to 15 |
| 30 days | Day 17 | Days 12 to 17 |
| 32 days | Day 19 | Days 14 to 19 |
| 35 days | Day 22 | Days 17 to 22 |
| 40 days | Day 27 | Days 22 to 27 |
| 45 days | Day 32 | Days 27 to 32 |
Getting the most from the estimate
- Ovulation moves. Stress, illness, travel and changes in weight can delay it, and cycles vary from month to month. Treat the dates as a guide.
- Narrow it down. Ovulation (LH) test strips, changes in cervical mucus and a rise in resting body temperature can help confirm when you ovulate.
- Regular sex covers the window. Because the exact day is uncertain, many couples simply have sex every two to three days rather than timing it to one date.
- Irregular cycles need a different approach. If your cycles are often shorter than 21 days or longer than 35, or you have been trying for a while, talk to a doctor.
Never use this calculator as contraception: an early ovulation and sperm that survive several days can lead to pregnancy outside the predicted window. This is not medical advice; talk to your doctor or midwife. For your period dates over the next year, use the period calculator; to plan for a particular due date, use the conception calculator.
Sources
- Wilcox AJ, Weinberg CR, Baird DD, "Timing of sexual intercourse in relation to ovulation", N Engl J Med 1995;333(23):1517–1521.
- NHS, Periods (reviewed January 2023).
- ACOG FAQ, Abnormal Uterine Bleeding (normal cycle length).
Frequently asked questions
How do I calculate when I ovulate?
Take the date your next period is due and count back the length of your luteal phase, usually about 14 days. If your period started on October 1, 2026 and your cycle is 28 days, the next is due October 29, so ovulation is estimated on October 15.
What is the fertile window?
The six days ending on the day of ovulation. In a 1995 study, Wilcox and colleagues found pregnancies only came from sex during those days. For ovulation on October 15, the fertile window is October 10 to 15.
When do I ovulate with a long or short cycle?
Ovulation is usually about two weeks before the next period, so it shifts with cycle length: around day 15 of a 28-day cycle, day 19 of a 32-day cycle and day 22 of a 35-day cycle, counting the first day of your period as day 1.
Can I use an ovulation calculator for contraception?
No. Ovulation can come earlier or later than predicted, even in regular cycles, and sperm can survive in the body for several days, so pregnancy is possible outside the estimated window. Talk to a doctor or nurse about reliable contraception.
What is the luteal phase?
The time from ovulation to the start of the next period. It is usually around 14 days; the NHS says ovulation happens about 12 to 16 days before a period. If tests have shown yours is shorter or longer, enter it to adjust the dates.