Ovulation Calculator
Enter the first day of your last period and your cycle length to see your estimated ovulation day, your fertile window (the days on which pregnancy is possible) and an ovulation calendar for the coming months.
- Fertile window30.10.2026 – 04.11.2026
- TodayIn your fertile window
- Cycle day of ovulation14
- Next periodThursday 19.11.2026
- Cycle length (FIGO)Normal (24–38 days)
Estimated ovulation calendar
How it is calculated
In a prospective study that detected ovulation from urinary hormones (Wilcox, Weinberg and Baird 1995; 221 women, 625 cycles), pregnancy followed only intercourse during the 6-day period ending on the day of ovulation; this is called the fertile window. The chance of conception was about 10% for intercourse 5 days before ovulation and about 33% on ovulation day itself; no pregnancies followed intercourse after ovulation. A calendar estimate assumes the luteal phase after ovulation has a fixed length: in a study of 327 luteal phases (Lenton et al. 1984), the normal luteal phase averaged 14.13 ± 1.41 days (counted from the day after the LH peak to the day before the period), and luteal phases of 9 days or less were abnormal. The calculator’s default of 14 days is instead the Naegele convention of due-date dating: ACOG Committee Opinion 700 assumes a regular 28-day cycle “with ovulation occurring on the 14th day”. Because Lenton counted from the day after the LH peak, and ovulation comes a median 16.5 hours after the LH peak (WHO Task Force 1980), a luteal phase counted from the day after ovulation may be about a day shorter. The tool therefore uses 14 days by default. Real cycles deviate from this: in more than 600,000 cycles recorded in an app (Bull et al. 2019) the luteal phase averaged 12.4 days, was 7–17 days in 95% of cycles and shorter than 11 days in about 18% of them. Ovulation day varies even more: in a study of 696 cycles (Wilcox, Dunson and Baird 2000), the fertile window of only about 30% of women fell entirely on cycle days 10–17; on every day from day 6 to day 21 the chance of being in the fertile window was at least 10%, and the authors stressed that the window can be unpredictable even in regular cycles. The results are therefore only an average estimate. Signs such as LH tests and basal body temperature track ovulation more closely. This page is for information only, is not medical advice and cannot be used to prevent pregnancy.
Ovulation = cycle day (Cycle length − Luteal phase) = Last period + (Cycle − Luteal − 1) days; Fertile window = 5 days before ovulation … ovulation day; Next period = Last period + CycleCycle length: days from the first day of one period to the first day of the next. Luteal phase: the days from the day after ovulation to the day before the next period; 14 days by default. Day 1 of the cycle is the day bleeding starts. For later cycles the cycle length is added to every date.- Subtract the luteal phase from the cycle length: the cycle day of ovulation (28 − 14 = day 14).
- Add that day minus one to the first day of the last period: the estimated ovulation date.
- Go back 5 days from ovulation: the first day of the 6-day fertile window.
- For later cycles add the cycle length to the dates; windows already past today are skipped.
Example
If the last period started on 18.10.2026, with a 28-day cycle and a 14-day luteal phase: ovulation is cycle day 14, i.e. 18.10.2026 + 13 days = Saturday 31.10.2026; the fertile window is 26.10.2026–31.10.2026 and the next period is due on 15.11.2026. In the following cycle ovulation is on 28.11.2026. With a 32-day cycle ovulation is day 18, 04.11.2026.
Frequently asked questions
Why is the fertile window 6 days?
In the study based on hormonally detected ovulation, pregnancies followed only intercourse on ovulation day and the 5 days before it, with the chance rising towards ovulation (from about 10% to 33%). No pregnancies followed intercourse after ovulation.
Does ovulation always happen on day 14?
No. Day 14 follows from assuming a 14-day luteal phase in a 28-day cycle. In longer cycles ovulation comes later and in shorter ones earlier, and it varies from cycle to cycle in the same person. In research, the fertile window of only about 30% of women fell entirely on days 10–17.
How can I know my luteal phase length?
If you follow ovulation for a few cycles with LH tests or basal body temperature, count the days from the day after ovulation to the day before the next period. If you don’t know it, keep the default of 14 days. The calculator accepts luteal phases longer than 9 days; if yours is consistently short, talk to your doctor.
Can I use this calendar for birth control?
No. The tool only gives an average calendar estimate; ovulation can come unexpectedly early or late, and days outside the window are not “safe”. Ask your doctor or a family planning service about birth control.
When should I see a doctor?
See your doctor if your cycle is regularly shorter than 24 or longer than 38 days (frequent or infrequent menstruation under FIGO) or very irregular, if your period is late and you could be pregnant, or if you have not conceived after 12 months of regular unprotected intercourse (the World Health Organization’s definition of infertility).
Source
- Wilcox AJ, Weinberg CR, Baird DD, Timing of sexual intercourse in relation to ovulation, N Engl J Med 1995;333(23):1517–21, doi:10.1056/NEJM199512073332301 — abstract, Results and Conclusions (6-day window, 10% and 33% probabilities)
- Wilcox AJ, Dunson D, Baird DD, The timing of the “fertile window” in the menstrual cycle, BMJ 2000;321(7271):1259–62, doi:10.1136/bmj.321.7271.1259 — abstract, Results and Conclusions (days 6–21; ~30% of women within days 10–17)
- Lenton EA, Landgren BM, Sexton L, Normal variation in the length of the luteal phase of the menstrual cycle, Br J Obstet Gynaecol 1984;91(7):685–9, doi:10.1111/j.1471-0528.1984.tb04831.x — abstract (luteal phase 14.13 ± 1.41 days; ≤9 days abnormal)
- ACOG, AIUM, SMFM — Committee Opinion No. 700, Methods for Estimating the Due Date, Obstet Gynecol 2017;129(5):e150–e154, doi:10.1097/AOG.0000000000002046: Background (regular 28-day cycle, “ovulation occurring on the 14th day”; the default luteal phase)
- WHO Task Force on Methods for the Determination of the Fertile Period. Temporal relationships between ovulation and defined changes in the concentration of plasma estradiol-17 beta, luteinizing hormone, follicle-stimulating hormone, and progesterone. I. Probit analysis. Am J Obstet Gynecol 1980;138(4):383–90 — abstract (median 16.5 hours from the LH peak to ovulation)
- Bull JR et al., Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles, NPJ Digit Med 2019;2:83, doi:10.1038/s41746-019-0152-7 — abstract (luteal phase 12.4 days, 7–17), Discussion (18% under 11 days), Methods (the luteal phase starts the day after ovulation); funded by Natural Cycles
- Munro MG et al. (FIGO), The FIGO ovulatory disorders classification system, Int J Gynaecol Obstet 2022;159(1):1–20, doi:10.1002/ijgo.14331 — section 6.1.1 (normal cycle 24–38 days)
- World Health Organization, Infertility (fact sheet), “Overview”: no pregnancy after 12 months of regular unprotected intercourse