Ovulation Calculator

What is an ovulation calculator?

An ovulation calculator estimates the day you are most likely to release an egg, and the fertile window around it, from the first day of your last menstrual period and your average cycle length. Ovulation usually happens about 14 days before your next period starts, because the luteal phase (the second half of the cycle) stays roughly constant while cycle length mostly varies in the first half. The fertile window spans the five days before ovulation plus the ovulation day itself, because sperm can survive several days in the reproductive tract. The tool projects the next three cycles so you can plan ahead, whether you are trying to conceive or simply tracking your cycle.

How to use it

1. Enter the first day of your last menstrual period. 2. Set your average cycle length (the number of days from one period to the next — 28 is the typical default). The calculator shows your estimated ovulation day, your fertile window, and the start of your next period, repeated across the next three cycles. If your cycles are irregular, treat the dates as approximate and consider tracking other signs such as basal body temperature or ovulation tests.

How it is calculated

Each cycle starts on the first day of a period. The next period is the cycle start plus the cycle length, and ovulation is estimated 14 days before that: Ovulation = next period − 14 days Fertile window = ovulation − 5 days through the ovulation day For a 28-day cycle starting on January 1, 2026, the next period falls on January 29, ovulation on January 15, and the fertile window runs January 10–15. The 14-day luteal phase is an average; your own may differ slightly.

Reading your results

The most fertile days are the two or three days leading up to and including ovulation. Intercourse during the fertile window gives the best chance of conception; if you are avoiding pregnancy, these are the days to be cautious. Because ovulation timing shifts with cycle length and stress, the estimate is a guide rather than a guarantee — many people ovulate a day or two earlier or later than predicted. Combining this calculator with ovulation predictor kits or basal body temperature charting gives a much more accurate picture.

Frequently asked questions