Estimate your most fertile days based on your last menstrual period and average cycle length.
How Ovulation Date Is Estimated
This calculator counts back 14 days from your next expected period to estimate ovulation, then adds your average cycle length to your last menstrual period (LMP) to find that next period date. The 14-day figure is the average length of the luteal phase (the time between ovulation and the next period), which is far more consistent across women than the time from period to ovulation.
What the Fertile Window Means
The fertile window covers the days around ovulation when conception is possible — typically the 5 days before ovulation plus the day of ovulation itself, since sperm can survive up to 5 days in the reproductive tract while the egg survives roughly 12–24 hours after release. Intercourse timed within this window carries the highest chance of conception.
Why Cycle Length Changes the Estimate
Ovulation timing shifts with cycle length because the luteal phase stays roughly fixed at 14 days while the follicular phase (period to ovulation) is what varies. A 35-day cycle pushes ovulation to around day 21, not day 14, which is why entering your actual average cycle length — not assuming 28 days — matters for accuracy.
How many days before my period do I ovulate?
Ovulation typically occurs 14 days before your next period, not 14 days after your last one — the luteal phase (ovulation to next period) is far more consistent than the follicular phase, which shifts with cycle length.
What is the fertile window?
The fertile window is the 5 days before ovulation plus the day of ovulation itself, since sperm can survive up to 5 days in the reproductive tract while the egg survives roughly 12–24 hours after release.