Ovulation date, fertile window and next period for the coming cycles, from the first day of your last period and your cycle length.
By AdminDrafted with AI writing toolsUpdated Checked examples: 5
With a 28-day cycle, ovulation is expected around Monday, 16 November 2026 (cycle day 15). The most fertile days run from Wed 11 Nov 2026 to Tue 17 Nov 2026, and the next period is due about Mon 30 Nov 2026.
| Cycle | Period starts | Fertile from | Ovulation | Fertile to | Next period |
|---|---|---|---|---|---|
| 1 | Monday, 2 November 2026 | Wednesday, 11 November 2026 | Monday, 16 November 2026 | Tuesday, 17 November 2026 | Monday, 30 November 2026 |
| 2 | Monday, 30 November 2026 | Wednesday, 9 December 2026 | Monday, 14 December 2026 | Tuesday, 15 December 2026 | Monday, 28 December 2026 |
| 3 | Monday, 28 December 2026 | Wednesday, 6 January 2027 | Monday, 11 January 2027 | Tuesday, 12 January 2027 | Monday, 25 January 2027 |
Five days before ovulation plus the day itself (sperm survival, Wilcox 1995), with one day after as a margin.
Ovulation is estimated by counting back a 14-day luteal phase from the next expected period, which is the last period's start plus the usual cycle length. The fertile window runs from 5 days before ovulation to 1 day after: Wilcox and colleagues (1995) found conception only followed intercourse in the six days ending on ovulation day, and one day is added as a margin. The same dates are projected over the coming cycles.
People use it to time intercourse when trying to conceive or to anticipate the next period. With a period starting on 5 January 2026 and a 28-day cycle, ovulation falls around 19 January, the fertile window runs from 14 to 20 January, and the next period is due on 2 February.
Calendar dates assume a regular cycle and a fixed luteal phase. In 612,613 app-recorded cycles (Bull and colleagues, 2019) the luteal phase averaged 12.4 days and varied widely, and the authors recommend tracking signs such as basal body temperature rather than cycle length alone. A calendar estimate is not reliable contraception on its own.
Add your cycle length to the first day of your last period, then count back 14 days. A period that began on 1 October 2026, in a 28-day cycle, puts the next one on 29 October and ovulation around 15 October. It assumes a regular cycle and a 14-day gap between ovulation and period.
Ovulation is the release of an egg from an ovary. Because sperm can survive for several days, the fertile window opens before it.
In Wilcox and colleagues' study of 221 women and 625 cycles, conception followed intercourse only in the six days ending on ovulation day. The chance rose from 10% five days before ovulation to 33% on the day itself.
The calculator adds the day after ovulation as a margin, so a 15 October estimate gives a window of 10 to 16 October.
Counting the first day of bleeding as cycle day 1, ovulation 14 days after a 1 October period falls on 15 October, which is cycle day 15. The familiar "day 14" means 14 days after the period started.
Ovulation is counted back from the next period, so a longer cycle moves it later by the same number of days. For a period that began on 1 October 2026, with a 14-day luteal phase:
| Usual cycle length | Estimated ovulation | Fertile window from | Fertile window to | Next period expected |
|---|---|---|---|---|
| 21 days | 8 Oct 2026 | 3 Oct 2026 | 9 Oct 2026 | 22 Oct 2026 |
| 28 days | 15 Oct 2026 | 10 Oct 2026 | 16 Oct 2026 | 29 Oct 2026 |
| 30 days | 17 Oct 2026 | 12 Oct 2026 | 18 Oct 2026 | 31 Oct 2026 |
| 35 days | 22 Oct 2026 | 17 Oct 2026 | 23 Oct 2026 | 5 Nov 2026 |
In the 21-day cycle the window opens on 3 October, cycle day 3, when bleeding may not have finished. With short cycles, sex at the end of a period or just after it can lead to pregnancy.
The time from ovulation to the next period is the luteal phase, 14 days in textbooks. In Bull and colleagues' study of 612,613 app-recorded ovulatory cycles, it averaged 12.4 days.
NHS guidance puts ovulation 12 to 16 days before the next period. In the 28-day example above, that is anywhere from 13 to 17 October.
With a 12-day luteal phase the estimate moves to 17 October and the window to 12–18 October. The next period stays on 29 October, because cycle length alone sets it.
Each later cycle repeats the same interval. A last period on 1 September 2026 means ovulation about 15 September and a next period due on 29 September; the cycle after that points to ovulation around 13 October.
The further ahead a projection reaches, the less it means, because real cycles vary in length. Recalculate from the first day of each new period. If a period is late and pregnancy is possible, take a pregnancy test.
A calendar projects dates from past cycles; it cannot show whether ovulation has happened. Bull's team recommends tracking signs such as basal body temperature as well as cycle length, and ACOG describes these fertility awareness methods.
Calendar estimates alone are not reliable contraception, so do not treat days outside the window as safe. Cycles shorter than 21 or longer than 35 days are outside the usual adult range and harder to predict.
For a due date from a conception date, IVF transfer or last period, use the pregnancy due date calculator. The period calculator lays out the next periods with the same arithmetic, and the conception calculator works back from a due date.
Checked against: Next = LMP + 28 d, ovulation = next − 14 d, window −5/+1 d (Wilcox 1995)
Checked against: 2026-01-01 + 35 − 14 = 2026-01-22
Checked against: 2026-01-10 + 21 − 14 = 2026-01-17
Checked against: Cycle 1 ovulation 2025-12-20 + 16 d; cycle 2 ovulation 2026-01-19 + 16 d; today 2026-01-15 is day 27
Around cycle day 15, counting the first day of the period as day 1, if the luteal phase is the textbook 14 days: ovulation falls 14 days before the next period. In a 35-day cycle that moves to about day 22, and in a 21-day cycle to about day 8. Real cycles vary: across 612,613 recorded cycles the average luteal phase was 12.4 days, which puts ovulation later than the textbook count.
About six days: the five days before ovulation and the day of ovulation. In Wilcox and colleagues' 1995 study of 221 women and 625 cycles, conception followed intercourse only in that window, and the chance of conception rose from 10 % five days before ovulation to 33 % on ovulation day itself. This calculator adds the day after ovulation as a margin, so the window shown is seven days.
Yes, especially with short cycles. Intercourse up to five days before ovulation can lead to conception (Wilcox 1995), so a 21-day cycle with a 14-day luteal phase puts ovulation around day 8 and the start of the fertile window around day 3, when bleeding may not have finished. With a 28-day cycle the window starts around day 10.
For information only, not medical advice. These formulas describe populations, not you; talk to a clinician before acting on a result.
This calculator includes 5 worked examples with independently sourced answers. They run in the test suite, and you can run them here too.