Ovulation is the day an egg is released — and only a handful of days around it are the one time in your cycle when conception can happen. This calculator estimates that day from your last period and average cycle length (ovulation lands about 14 days before the next period), then maps your fertile window: the six days ending on ovulation day when intercourse is most likely to lead to pregnancy.
The calculator works backwards from your next period. The second half of the cycle — the luteal phase — is remarkably constant at about 14 days, so ovulation is estimated as your cycle length minus 14. On a 28-day cycle that’s around day 14; on a 32-day cycle, around day 18. It’s the first half of the cycle that varies, which is exactly why longer or shorter cycles shift ovulation later or earlier.
Pregnancy is only possible during the fertile window — six days ending on ovulation day. The range exists because sperm can survive in the body up to five days, while the egg lives just 12–24 hours after release. The two days before ovulation carry the highest chance of all. Calendar estimates are a starting point, not a guarantee: stress, illness or travel can move ovulation. For more certainty, combine this with ovulation tests, basal body temperature or cervical-fluid tracking.