Ovulation & Fertility Window Calculator

Understanding your menstrual cycle

A typical menstrual cycle lasts between 21 and 35 days, with 28 days being the average. Each cycle begins on the first day of your period and ends the day before your next period starts. The cycle has two main phases:

• Follicular phase (days 1–13 in a 28-day cycle): Begins with menstruation. Follicle-stimulating hormone (FSH) prompts multiple follicles in the ovaries to develop, though usually only one becomes dominant. Rising estrogen causes the uterine lining to thicken. This phase is variable in length — it's the main reason cycle length varies between people.
• Luteal phase (days 15–28): Begins after ovulation. The ruptured follicle becomes the corpus luteum, which produces progesterone to maintain the uterine lining. This phase is remarkably consistent at approximately 14 days in most people. If fertilization does not occur, the corpus luteum regresses, progesterone drops, and menstruation begins.

When does ovulation occur?

Ovulation — the release of a mature egg from the dominant follicle — is triggered by a surge in luteinizing hormone (LH). This LH surge typically occurs 24–36 hours before ovulation. Because the luteal phase is consistently ~14 days, ovulation occurs approximately14 days before the start of your next period, not necessarily on day 14 of your cycle.

For a 28-day cycle: ovulation ≈ day 14. For a 32-day cycle: ovulation ≈ day 18. For a 24-day cycle: ovulation ≈ day 10. This calculator uses your cycle length to estimate your individual ovulation day.

The fertile window: what it is and how to use it

The fertile window is approximately six days long: the five days leading up to ovulation and the day of ovulation itself. This window exists because sperm can survive in the female reproductive tract for up to five days under favorable conditions, while the egg remains viable for only 12–24 hours after release.

Research published in the New England Journal of Medicine (Wilcox et al., 1995) found that the highest probability of conception came from intercourse one or two days before ovulation. Intercourse on the day of ovulation also carries a high probability. The probability of conception drops sharply after ovulation day.

How to detect ovulation at home

Ovulation predictor kits (OPKs) detect the LH surge in urine 24–36 hours before ovulation. Start testing a few days before your estimated ovulation date (we show this as your "test start" date). A positive OPK means ovulation is likely within the next 24–36 hours.

Basal body temperature (BBT) tracking involves taking your temperature each morning before getting out of bed. After ovulation, progesterone causes a sustained temperature rise of 0.2–0.5°F (0.1–0.3°C). This confirms ovulation has already occurred and, over several cycles, can help predict your pattern.

Cervical mucus monitoring: In the days approaching ovulation, cervical mucus becomes increasingly clear, slippery, and stretchy — resembling raw egg whites. This is called "spinnbarkeit." Its presence is a sign of peak fertility. After ovulation, mucus typically becomes thicker and less abundant.

When to seek help

ASRM recommends evaluation if you've been trying to conceive for 12 months(or 6 months if you are 35 or older) with regular, unprotected intercourse. Seek evaluation sooner if you have irregular cycles, a history of pelvic inflammatory disease, prior surgeries, or other risk factors. Both partners should be evaluated — male factor contributes to infertility in approximately 40–50% of cases.