Ovulation Calculator
Estimate your most probable ovulation / fertile window and the related dates — the intercourse window, the earliest sensible pregnancy-test day, your next period, and the due date if you conceive — from the first day of your last period, projected over six cycles. This calculator should not be used as a form of birth control.
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Ovulation and conception
Ovulation is the moment an ovarian follicle ruptures and releases one or more mature eggs. On average it falls within the four days before or after the cycle's midpoint — equivalently, about 14 days before the next period begins. Days 10–18 of the cycle are typically its most fertile phase. Timing matters because the egg can be fertilized for only 12–24 hours after release as it travels through the fallopian tube toward the uterus, while sperm can survive inside the body for up to five days — so regular sex across the five days before ovulation, and on the day itself, gives the best chance of conception. A fertilized egg implants in the uterus 6–12 days later; if there is no fertilization, menstruation follows and the cycle restarts.
Predicting ovulation
Tracking menstrual cycles — the method this calculator uses. Day one of a cycle is the day bleeding starts, and the cycle ends the day the next bleed begins. With regular cycles, the fertile phase is easy to locate; keeping a menstrual calendar for a few months shows how regular you actually are. With irregular cycles, calendar math alone is unreliable and the other methods earn their keep. Basal body temperature (BBT) — your temperature on waking is at its lowest just before ovulation, then rises about half a degree as ovulation happens; charting it over several months reveals your personal pattern, though a cold or infection can distort a reading. Ovulation kits — over-the-counter tests detect the surge in luteinizing hormone (LH) that precedes ovulation by 24–48 hours; they are about 99% accurate at detecting the hormone itself, though they can't promise the exact hour of release. Other kits track estrogen in saliva or salts in sweat, which shift earlier in the cycle but require baseline tracking first.