Ovulation date, fertile window and next period for the coming cycles, from the first day of your last period and your cycle length.
With a 28-day cycle, ovulation is expected around Thursday, 1 October 2026 (cycle day 15). The most fertile days run from Sat 26 Sep 2026 to Fri 2 Oct 2026, and the next period is due about Thu 15 Oct 2026.
| Cycle | Period starts | Fertile from | Ovulation | Fertile to | Next period |
|---|---|---|---|---|---|
| 1 | Thursday, 17 September 2026 | Saturday, 26 September 2026 | Thursday, 1 October 2026 | Friday, 2 October 2026 | Thursday, 15 October 2026 |
| 2 | Thursday, 15 October 2026 | Saturday, 24 October 2026 | Thursday, 29 October 2026 | Friday, 30 October 2026 | Thursday, 12 November 2026 |
| 3 | Thursday, 12 November 2026 | Saturday, 21 November 2026 | Thursday, 26 November 2026 | Friday, 27 November 2026 | Thursday, 10 December 2026 |
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.
Checked against: Python 3.8 datetime: next = LMP + 28 d, ovulation = next − 14 d, window −5/+1 d (Wilcox 1995)
Checked against: Python 3.8 datetime: 2026-01-01 + 35 − 14 = 2026-01-22
Checked against: Python 3.8 datetime: 2026-01-10 + 21 − 14 = 2026-01-17
Checked against: Python 3.8 datetime: 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.
Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 5 worked examples whose answers come from independent sources; for example, “28-day cycle, period started 5 Jan 2026” is checked against Python 3.8 datetime: next = LMP + 28 d, ovulation = next − 14 d, window −5/+1 d (Wilcox 1995).
Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517–21; ACOG — Fertility awareness-based methods of family planning (FAQ).
For information only, not medical advice. These formulas describe populations, not you; talk to a clinician before acting on a result.
5 worked examples with independently sourced answers ship with this calculator. They run in the test suite; you can run them here too.
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