Ovulation typically happens about 14 days before your next period starts, regardless of how long your overall cycle runs. This calculator uses that pattern, along with your average cycle length, to estimate your ovulation day, fertile window, and next expected period.
The luteal phase, the time between ovulation and the next period, is fairly consistent at around 14 days, even when overall cycle length varies. This calculator adds your average cycle length to the first day of your last period to estimate your next period, then counts back 14 days from that to estimate ovulation. The fertile window is set from five days before ovulation to one day after, reflecting typical sperm survival time and the egg's short viable window.