Worked example
Your last period started on Thursday, October 1, 2026 and your cycle usually lasts 28 days.
Estimated next period: October 1 + 28 days = Thursday, October 29, 2026. Estimated ovulation: 28 − 14 = cycle day 14, so Wednesday, October 14. Estimated fertile window: October 9 to 15.
With a 32-day cycle and the same start date, ovulation would be estimated on day 18 (October 18): cycle length moves ovulation, there is no fixed “day 14” rule.
How it is calculated
- Take the first day of your last period (cycle day 1).
- Add your usual cycle length: that is your estimated next period.
- Count back 14 days from that date: that is your estimated ovulation day (cycle length − 14).
- Your fertile window runs from 5 days before ovulation to the day after ovulation.
How the ovulation day is estimated
After ovulation, the corpus luteum makes progesterone during the luteal phase, which ends with your period. That phase is fairly stable from cycle to cycle, around 14 days on average. That is why ovulation is calculated backwards from the next period: cycle length − 14.
The part that varies most is the phase before ovulation (the follicular phase). So a 35-day cycle does not ovulate “in the middle”, but closer to day 21.
| Cycle length | Estimated ovulation | Fertile window (cycle days) |
|---|---|---|
| 21 days | day 7 | days 2 to 8 |
| 24 days | day 10 | days 5 to 11 |
| 26 days | day 12 | days 7 to 13 |
| 28 days | day 14 | days 9 to 15 |
| 30 days | day 16 | days 11 to 17 |
| 32 days | day 18 | days 13 to 19 |
| 35 days | day 21 | days 16 to 22 |
Why the fertile window is about 6 days long
The egg only lives about 12 to 24 hours after ovulation, but sperm can survive up to about 5 days in the reproductive tract. A large prospective study found that almost all pregnancies followed intercourse in the 6 days ending on the day of ovulation, with the last two or three being the most fertile.
The calculator also includes the day after ovulation as a margin, like the Luteal app, because the ovulation date itself is uncertain.
Why it is only an estimate
- In a landmark study, only about 30% of women had their fertile window entirely between cycle days 10 and 17: for many, it comes earlier or later, even with regular cycles.
- An analysis of more than 600,000 cycles measured an average luteal phase of 12.4 days, with real differences between people: the 14-day rule is an average.
- Stress, illness, travel, weight change or intense training can shift ovulation from one month to the next.
- A calculation cannot tell whether ovulation actually happened.
Irregular cycles, PCOS, postpartum, contraception
- Irregular cycles (more than 7 to 9 days of difference between cycles): an average does not predict ovulation well. Ovulation tests or body signs tell you more.
- PCOS: ovulation may be infrequent or absent in some cycles; this calculation does not fit.
- After giving birth and while breastfeeding: ovulation can return before the first period, so without warning.
- Hormonal contraception (pill, implant, hormonal IUD, patch, ring): ovulation is usually stopped or changed; bleeding on the pill is not a real period. The calculation does not apply.
- Recently stopping the pill and perimenopause: cycles are often irregular for a while.
Pinpointing ovulation more precisely
Logging your periods and these signs over a few months gives a far better picture of your own rhythm than an average.
- Urine ovulation tests (pharmacy): they detect the LH surge that comes about 24 to 36 hours before ovulation.
- Cervical mucus: close to ovulation it becomes more abundant, clear and stretchy, like raw egg white.
- Waking temperature: it rises slightly (a few tenths of a degree) after ovulation, which confirms it afterwards.
When to see a doctor
- You have been trying to conceive for 12 months without success (or 6 months if you are 35 or older): a fertility check-up is recommended, for both partners.
- Your cycles are very irregular, or repeatedly shorter than 21 days or longer than 35 days.
- You have not had a period for more than 3 months and are not pregnant.
- Your periods are very painful or very heavy, or you bleed between periods or after sex.