Health
Ovulation Calculator
Your likely ovulation day, fertile window and most fertile days from your last period and cycle length.
Updated: NHS · Wilcox 1995
Result
Updates as you typeEstimated ovulation (Monday)
23 November 2026
- Fertile window
- 18 November 2026 – 23 November 2026
- Most fertile days
- 21 November 2026 – 23 November 2026
- Next period / test day
- 7 December 2026
- Cycle 2 ovulation
- 21 December 2026 (fertile 16 December 2026 – 21 December 2026)
How it's calculated
The phase after ovulation lasts about 14 days in most women; cycles differ in length mainly before ovulation. That is why ovulation is estimated by counting back from the next period, not forward from the last one. The fertile window starts 5 days before ovulation and ends on the day of ovulation. The calculator shows three cycles, starting with the one you are in now.
Worked example
Last period started 12 October 2026, 28-day cycle:
- Next period: 9 November 2026
- Ovulation: 9 November 2026 − 14 days = 26 October 2026
- Fertile window 21 October 2026 – 26 October 2026; most fertile 24 October 2026 – 26 October 2026
Ovulation day by cycle length
| Cycle | Ovulation | Fertile window |
|---|---|---|
| Cycle: 21 days | Ovulation7 days after | Fertile window2–7 days after |
| Cycle: 24 days | Ovulation10 days after | Fertile window5–10 days after |
| Cycle: 26 days | Ovulation12 days after | Fertile window7–12 days after |
| Cycle: 28 days | Ovulation14 days after | Fertile window9–14 days after |
| Cycle: 30 days | Ovulation16 days after | Fertile window11–16 days after |
| Cycle: 32 days | Ovulation18 days after | Fertile window13–18 days after |
| Cycle: 35 days | Ovulation21 days after | Fertile window16–21 days after |
Frequently asked questions
When do I ovulate?
On average 14 days before your next period; the NHS says around 10 to 16 days before it in most women. In a regular 28-day cycle that is about 14 days after your period starts: a period starting 12 October 2026 puts ovulation around 26 October 2026.
What is the fertile window?
The 5 days before ovulation and the day of ovulation itself, six days in all. Sperm can survive for several days, so sex in that window can lead to pregnancy. The chance is highest in the 2 days before ovulation and on the day itself (Wilcox and colleagues, 1995).
Does it work if my cycle is irregular?
The calendar method assumes every cycle is the same length. If yours varies by a few days, work it out with your shortest and your longest cycle and treat both windows as fertile. Ovulation tests (LH tests) and tracking your basal body temperature give a more precise picture.
When should I take a pregnancy test?
Home pregnancy tests are most reliable from the first day of a missed period. Testing earlier can give a negative result even if you are pregnant; if in doubt, test again a few days later.
Can I use this as contraception?
No. The calendar method on its own is not a reliable way to avoid pregnancy, because ovulation can shift from month to month even in the same person. Speak to a doctor, nurse or sexual health clinic about contraception.
Sources
- NHS — Fertility in the menstrual cycle: ovulation happens around 10 to 16 days before the next period in most women; sperm can survive up to 7 days
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333:1517-1521 (a six-day fertile window ending on the day of ovulation)
A calendar estimate; not a diagnosis, a fertility assessment or a method of contraception. Irregular cycles, breastfeeding, hormonal treatment and illness can change ovulation. Results are for guidance only.