Ovulation Calculator

Estimate your ovulation date and fertile window based on cycle length and last menstrual period. Plan or prevent pregnancy with cycle tracking.

This calculator applies the standard calendar method: it assumes a fixed 14-day luteal phase, so ovulation is estimated at your cycle length minus 14 days after the first day of your last period — day 14 of a 28-day cycle, but day 21 of a 35-day cycle. Cycle Length is the input that shifts every date: each extra cycle day pushes ovulation, the fertile window, and the next expected period out by exactly one day. The fertile window runs from five days before ovulation through one day after, reflecting sperm survival of up to five days plus a day or so of egg viability — seven days in total (five days before, ovulation day itself, and one day after). Last Period Day and Month anchor the calendar; the engine rolls forward across month boundaries using built-in month lengths (February is always counted as 29 days, so dates near the end of February can land a day off in non-leap years, and no year is ever considered). What the calendar method cannot know: whether you actually ovulated that cycle, luteal phases that differ from the assumed 14 days, or irregular cycles — every estimate scales with the average length you enter, and a cycle that varies by several days makes the window unreliable as contraception.