Ovulation & Fertility Calculator

Predict your most fertile days, ovulation date, and estimated due date. Features Shettles Method gender insights and a cycle probability log for better planning.

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About

Timing is everything when trying to conceive. The Ovulation & Fertility Calculator is designed to demystify the menstrual cycle, helping prospective parents identify the specific window where conception is statistically most likely. Unlike generic trackers, this tool considers sperm survival rates (up to 5 days) and the luteal phase to map out a precise 6-day fertile window.

We also include a fun feature based on the Shettles Method, a popular theory suggesting that timing intercourse relative to ovulation may influence the gender of the baby (though this is not scientifically guaranteed). Whether you are actively planning a pregnancy or just monitoring your reproductive health, this calculator provides a comprehensive daily breakdown of conception probabilities, helping you make informed decisions without the stress of manual counting.

fertility ovulation

Formulas

The calculation relies on the standard model of the menstrual cycle, where ovulation typically occurs 14 days before the start of the next period. The formulas used are:

  • Next Period = LMP + Cycle Length
  • Ovulation Date = Next Period − 14 days
  • Fertile Window Start = Ovulation Date − 5 days
  • Due Date = Ovulation Date + 266 days (38 weeks)

    • Note: The standard luteal phase is 14 days, but can range from 10 to 16 days. This tool uses the 14-day standard average.

    Reference Data

    Cycle Day relative to OvulationConception ProbabilityShettles Method Theory (Gender)Physiological Event
    Ovulation Day - 5~0-5%N/ASperm may survive, but rare.
    Ovulation Day - 4~10-12%Girl (X-sperm live longer)Sperm awaits egg in fallopian tube.
    Ovulation Day - 3~15-20%Girl FavoredEstrogen peaks; cervical mucus changes.
    Ovulation Day - 2~25-30%Girl/Boy MixPeak fertile window begins.
    Ovulation Day - 1~30-35%Boy Favored (Y-sperm are faster)LH Surge triggers ovulation.
    Ovulation Day (0)~30-40%Boy (Highest chance)Egg release; viable for 12-24h.
    Ovulation Day + 1~0-5%N/AEgg likely disintegrated; window closes.
    Luteal Phase0%N/AProgesterone rises; uterine lining thickens.

    Frequently Asked Questions

    The gender prediction is based on the Shettles Method, a theory from the 1960s suggesting Y-chromosome sperm (male) are faster but shorter-lived, while X-chromosome sperm (female) are slower but more resilient. While popular, modern scientific studies show mixed results. It should be used for entertainment and general planning rather than a medical guarantee.
    If your cycle varies significantly (e.g., 25 days one month, 35 the next), relying solely on a calendar calculator is difficult. In such cases, it is recommended to track your Basal Body Temperature (BBT) or use ovulation predictor kits (OPKs) alongside this calculator to pinpoint your exact ovulation day.
    While the egg remains viable for only 12-24 hours after release, sperm can survive inside the female reproductive tract for up to 5 days. This means intercourse that happens days before ovulation can still result in conception once the egg is released.
    The 'Naegele’s Rule' used here is the medical standard. However, only about 4-5% of babies are born on their exact due date. Most are born within the two weeks before or after that date.