Ovulation and Fertile Window Calculator
Estimate your ovulation day, fertile window (most fertile days) and next period from the first day of your last period and your cycle length.
Things to keep in mind:
- The calendar method is an estimate; ovulation can shift by several days from cycle to cycle even in the same woman. It is not a reliable method of contraception.
- Results can be misleading with irregular cycles, while breastfeeding, after birth or miscarriage, on hormonal medication or with conditions such as polycystic ovary syndrome.
- If you have trouble conceiving (12 months of regular intercourse under age 35, 6 months at 35 or older), see a gynaecologist.
- The result is an estimate, not a medical diagnosis or treatment advice; consult your physician or dietitian for decisions about your health.
Calculator
Result
Enter the values and press Calculate to see the result here.
About this tool
Ovulation usually occurs about 14 days before the next period, because the luteal phase after ovulation is much more constant than the follicular phase. Since sperm can survive for several days in the female reproductive tract, the fertile window in which pregnancy is possible is taken as the 6 days made up of the 5 days before ovulation and the day of ovulation (Wilcox et al., 1995).
Formulas
- Next period = first day of last period + cycle length
- Ovulation day = next period − luteal phase length (default 14 days)
- Fertile window = ovulation − 5 days … ovulation day
- Ovulation in the following cycle = ovulation + cycle length
This calendar method gives an average-based estimate for regular cycles. Basal body temperature tracking, cervical mucus observation or LH (ovulation) tests show actual ovulation more accurately.
Example: with a last period on 1 September 2026 and a 28-day cycle, the next period is due on 29 September. Ovulation is 29 September − 14 days = 15 September 2026 and the fertile window runs from 10 to 15 September. Ovulation in the following cycle is estimated for 13 October 2026.
How to use
- Select the first day of your last period.
- Enter your average cycle length.
- Enter your luteal phase length if you know it (otherwise leave 14).
- Press Calculate.
Notes
Dates are calendar days without time zones. The fertile window is 6 days including its first and last day. Estimates for the following cycle assume your cycle length stays the same.
Sources
References
- [1]Timing of Sexual Intercourse in Relation to Ovulation — Effects on the Probability of Conception, Survival of the Pregnancy, and Sex of the BabyWilcox AJ et al., N Engl J Med, 1995
Frequently asked questions
Pregnancy is possible in the 5 days before ovulation and on the day of ovulation; the chance is highest on the 2 days before ovulation and on the day itself.
It is the time from ovulation to the next period. For most women it lasts 12–16 days, 14 on average; differences in cycle length mostly come from the phase before ovulation.
You can enter the average of your last few cycles, but the result is less reliable. LH tests or basal body temperature tracking give better information.
No. A calendar estimate cannot predict a shift in ovulation; agree on a reliable method with your doctor.
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