How the fertile window is estimated
A menstrual cycle has two halves. The follicular phase, from the start of a period to ovulation, varies in length from person to person and month to month. The luteal phase, from ovulation to the next period, is steadier — about 14 days. So the calculator finds your next expected period from your cycle length and counts back by the luteal phase to estimate ovulation. Because sperm can survive for several days, the fertile window starts five days before that.
The formulas
Next period = last period + cycle length Ovulation = next period − luteal phase (cycle day = cycle − luteal) Fertile window = ovulation − 5 days … ovulation day Peak days = ovulation − 2 days … ovulation day
Worked example
Last period began October 5, 2026, cycle 28 days, luteal phase 14 days. Next period: November 2. Ovulation: November 2 − 14 = October 19 — cycle day 15, counting the first day of bleeding as day 1.
Fertile window: October 14 to October 19, with October 17–19 the most fertile. The next cycle's window is November 11–16.
With a 32-day cycle, ovulation moves to October 23.
Limitations
- Predictions assume regular cycles; real ovulation can shift by several days in any month, even in regular cycles.
- Not a contraceptive method and not a substitute for ovulation tests or medical advice.
- Conditions such as PCOS, thyroid disorders, recent childbirth, breastfeeding or stopping hormonal contraception change cycle timing.
Sources: Wilcox AJ, Weinberg CR, Baird DD, "Timing of sexual intercourse in relation to ovulation," N Engl J Med 1995;333:1517–1521; American College of Obstetricians and Gynecologists, Fertility Awareness-Based Methods of Family Planning.
Frequently asked questions
When am I most fertile?
In the six days ending on the day of ovulation. In the landmark Wilcox study (NEJM, 1995), conception happened only when intercourse took place in that window, with the highest chances on the two days before ovulation and the day itself. Sperm can survive up to about five days; the egg lives for about a day.
How is ovulation day estimated?
By counting back from the next expected period. The luteal phase — the time from ovulation to the next period — is fairly stable at about 14 days for most people, while the phase before ovulation varies. So ovulation ≈ next period start − 14 days. In a 28-day cycle that is day 14; in a 32-day cycle, day 18.
Can I use this as birth control?
No. Calendar predictions can be off by several days, and cycles vary month to month. Fertility-awareness methods used for contraception combine daily signs such as basal body temperature and cervical mucus, and even then need training; talk to a clinician about reliable options.
What if my cycles are irregular?
Then a calendar estimate is much less reliable. Ovulation predictor kits (which detect the LH surge), basal body temperature charting or cervical-mucus tracking pick up ovulation as it happens. If cycles are often shorter than 21 or longer than 35 days, mention it to your doctor.
Should I change the luteal phase length?
Only if you know yours — for example from temperature charting. Most people's luteal phase is 12 to 16 days. Entering a known value makes the estimate more accurate.
This calculator is for general information and screening only and is not medical advice. Talk to a doctor or registered dietitian before making changes based on the result, especially if you are pregnant, under 18, or managing a health condition.