Ovulation Calculator
Predict peak fertile days, estimated ovulation date, and upcoming menstrual windows.
Ovulation & Fertility
| Period starts | Fertile window | Ovulation |
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Medical disclaimer. This calculator is for general information and education only. It is not medical advice, not a diagnosis, and not a substitute for professional care. Predicted fertile windows assume regular cycles and are not a contraceptive method. Ovulation timing shifts between cycles and sperm can survive for several days.Speak to a doctor or another qualified clinician about any health decision.Last reviewed .
Frequently asked questions
When does ovulation occur?
Ovulation typically occurs around 14 days before the start of your next menstrual period in a standard 28-day cycle.
When am I most likely to ovulate?
Ovulation typically occurs about 14 days before your next period starts, not 14 days after the last one began. That distinction matters: the luteal phase is relatively fixed at 12-16 days, while the follicular phase varies, so cycle length changes shift ovulation later or earlier.
How long is the fertile window?
About six days: the five days before ovulation plus ovulation day itself. Sperm can survive up to five days in fertile cervical mucus, while the egg is viable for only 12-24 hours. Conception probability peaks in the two days before ovulation.
How reliable is calendar-based prediction?
Only moderately. Studies tracking ovulation with hormone testing find that even women with regular cycles ovulate outside the predicted window a substantial share of the time, and stress, illness and travel shift timing. Basal body temperature, cervical mucus observation and LH test strips are all more reliable. Do not rely on calendar prediction as contraception.