Estimate your ovulation date, fertile window and next expected period based on the first day of your last period and your average cycle length.
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This calculator estimates ovulation as occurring 14 days before your next expected period, which is calculated by adding your average cycle length to the first day of your last period. This is the standard method used since the second half of the menstrual cycle (the luteal phase) is typically a consistent 14 days, regardless of total cycle length.
The fertile window spans from about 5 days before ovulation through 1 day after, since sperm can survive in the reproductive tract for up to 5 days while an egg is typically viable for about 24 hours after release. Conception is most likely for intercourse occurring in the 1-2 days leading up to and including ovulation day itself.
For example, with a last period starting June 1, 2026 and a 28-day average cycle: the next period is expected June 29, 2026 (June 1 + 28 days), so ovulation is estimated 14 days before that, on June 15, 2026. The fertile window then runs from June 10 (5 days before ovulation) through June 16 (1 day after).
Using an average or 'textbook' 28-day cycle length instead of your own actual average, cycle length varies significantly between individuals and directly shifts the estimated ovulation date.
Relying on this estimate alone for contraception, ovulation timing naturally varies cycle to cycle, this tool is intended for planning and awareness, not as a birth control method.
Ignoring cycle irregularity, if cycles vary widely month to month, a single average-based estimate is much less reliable than tracking methods like basal body temperature or ovulation predictor kits.
This calculator provides a general estimate based on average cycle patterns. Actual ovulation timing varies cycle to cycle and person to person, especially with irregular cycles. Ovulation predictor kits or tracking basal body temperature can provide more personalized data.
Use your average cycle length for the best estimate, but be aware that the actual ovulation date may shift if a given cycle is shorter or longer than your average. Tracking several cycles can help identify your typical pattern.
No, this tool is intended for general planning and awareness only. It should not be relied on as a method of contraception, since ovulation timing can vary and this estimate isn't precise enough for that purpose.
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