Predicted ovulation date, fertile window and next period, from your last period and cycle length.
It assumes a 14-day luteal phase (the time from ovulation to your next period), so ovulation day = LMP + (cycle length − 14). For a 28-day cycle that puts ovulation around day 14.
The fertile window shown is the 5 days before ovulation through 1 day after it — reflecting that sperm can survive a few days and the egg is viable for roughly a day after release.
This calculator uses a single average cycle length you enter (20–45 days), assuming a constant 14-day luteal phase. If your cycles are irregular, treat the predicted dates as a rough window rather than an exact day.