Calculator
Estimate your likely ovulation day and fertile window from your last period's start date and average cycle length.
Ovulation typically happens about 14 days before the next period starts, regardless of overall cycle length — so this calculator estimates it as (cycle length − 14) days after the start of the last period, then marks a fertile window in the days leading up to it, since sperm can survive in the body for several days before an egg is released.
Actual ovulation timing varies cycle to cycle, even for people with very regular periods, and this calendar-based method is meaningfully less accurate than tracking methods like basal body temperature, ovulation predictor kits, or cervical mucus changes. Treat this as a general awareness tool, not a reliable method for either achieving or avoiding pregnancy — talk with a healthcare provider for guidance suited to your specific situation.
Related
Estimate a due date once a cycle results in pregnancy.
The general-purpose version of this same date math.
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