Ovulation calculator
Ovulation usually happens about 14 days before your next period starts, so the calculator finds your next expected period (first day of your last period plus your cycle length) and counts back 14 days. Your fertile window is the six days ending on ovulation day: the five days before it and the day itself. For a 28-day cycle that started on 3 November 2026, ovulation is estimated for 17 November 2026 and the fertile window for 12 November to 17 November 2026. These are estimates, not birth control.
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Worked example
Your last period started on 3 November 2026 and your cycles are usually 28 days long.
- Next period expected: 3 November 2026 + 28 days = 1 December 2026.
- Estimated ovulation: 1 December 2026 − 14 days = 17 November 2026.
- Fertile window: the five days before ovulation plus ovulation day = 12 November to 17 November 2026.
- The Lunation app also marks 18 November 2026, the day after ovulation.
The same rule for other cycle lengths, starting from 3 November 2026:
| Cycle length | Estimated ovulation | Fertile window | Next period |
|---|---|---|---|
| 24 days | 13 November (cycle day 11) | 8 November – 13 November | 27 November |
| 28 days | 17 November (cycle day 15) | 12 November – 17 November | 1 December |
| 32 days | 21 November (cycle day 19) | 16 November – 21 November | 5 December |
| 35 days | 24 November (cycle day 22) | 19 November – 24 November | 8 December |
How the calculation works
The time from ovulation to the next period is the most consistent part of the cycle (ACOG), while the time before ovulation varies a lot from person to person and cycle to cycle. That is why the calculator works backwards from your next expected period rather than forwards from your last one. Health sources often round this to “around day 14” of a 28-day cycle; counting exactly 14 days back from day 29 (the next period) puts it on day 15, which is the convention the Lunation app uses. The NHS describes ovulation as usually happening 10 to 16 days before the next period, so treat the date as the middle of a range.
The fertile window is the six days that end on ovulation day. According to ACOG, sperm can survive in the body for about 3 and sometimes up to 5 days, while the egg survives about 24 hours, so sex in the days before ovulation can lead to pregnancy. The calculator shows the next three windows that have not ended yet; if today is past this cycle’s ovulation day, it starts with the next cycle.
Six days or seven? This calculator uses the six-day window from Wilcox and colleagues. The Lunation app shades one more day, the day after ovulation, in line with ACOG’s description that pregnancy is possible “until 1 day after ovulation”, because the exact ovulation day is never certain. The results name that extra day so you can compare the two.
How accurate is this?
In the 1995 Wilcox study in the New England Journal of Medicine, 221 women trying to conceive collected daily urine samples so ovulation could be measured. Conception happened only when sex took place in the six days ending on the estimated ovulation day, with the chance per day rising from about 10% five days before to about 33% on ovulation day.
The difficulty is knowing which day that is. A follow-up study by the same team found that only about 30% of women had their whole fertile window inside days 10 to 17, and that it can be unpredictable even in regular cycles. In Bull and colleagues’ analysis of 612,613 cycles, the phase before ovulation averaged 16.9 days but varied widely, while the phase after it averaged 12.4 days. A calendar estimate is a useful starting point for planning; signs such as cervical mucus and LH tests tell you more about a given cycle. None of it is a method of contraception.
Frequently asked questions
Can I use the fertile window as birth control?
No. Calendar estimates from a calculator or an app are not a method of contraception, because ovulation can come earlier or later than predicted. If you want to avoid pregnancy, talk to a doctor, nurse or midwife about contraception.
Why does the calculator not just say day 14?
Day 14 is a rounded textbook figure for a 28-day cycle. The calculator counts 14 days back from your next expected period instead, so the date moves with your cycle length: around day 15 of a 28-day cycle, or day 22 of a 35-day cycle.
Can I get pregnant outside the fertile window?
It is less likely, but possible, because your actual ovulation day can differ from the estimate. In a prospective study of 221 women, only about 30% had their fertile window entirely within days 10 to 17 of the cycle.
Why does the Lunation app shade seven days, not six?
The app also marks the day after ovulation, following ACOG’s description that pregnancy is possible from five days before ovulation until one day after. This calculator shows the six days from the Wilcox study, which is when nearly all conceptions happened, and names that extra day separately.
How can I tell when I actually ovulate?
Body signs narrow it down: clear, stretchy cervical mucus, a positive LH (ovulation) test, and a small rise in resting temperature after ovulation. Combined with your cycle dates, they give a closer estimate than a calendar alone.
Sources
- Timing of sexual intercourse in relation to ovulation (Wilcox, Weinberg and Baird, 1995). New England Journal of Medicine (via PubMed).
- The timing of the “fertile window” in the menstrual cycle (Wilcox, Dunson and Baird, 2000). BMJ (via PubMed Central).
- Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles (Bull et al., 2019). npj Digital Medicine.
- Fertility Awareness-Based Methods of Family Planning (FAQ). American College of Obstetricians and Gynecologists (ACOG).
- Periods and fertility in the menstrual cycle. NHS.
Written by the Lunation editorial team.
Related guides
- When Am I Most Fertile?Your most fertile days are the six ending on ovulation day. How to find them in your own cycle, why day 14 is only a rough guide, and when to talk to a doctor.
- What Are the Signs of Ovulation?Stretchy cervical mucus, a positive LH test and a small rise in resting temperature are the clearest signs of ovulation. What each one tells you, and when.
- What Are the Phases of the Menstrual Cycle?The four phases of the menstrual cycle explained: typical days, the hormones behind each one, and why real cycles rarely follow the 28-day textbook.
More calculators
- Period calculatorEstimate your next six periods, with start and end dates, from your last period, cycle length and period length.
- Due date calculatorEstimate a due date, current pregnancy week and trimester from your last period (280 days) or conception date (266 days).
- Cycle length calculatorWork out your average cycle length, shortest and longest cycle, and how much it varies from your period start dates.
Medical disclaimer. This calculator gives general wellness estimates, not medical advice. It does not diagnose or treat any condition and is not a method of contraception. If something about your cycle or your health worries you, talk to a doctor or midwife.