How the fertile window is worked out
A menstrual cycle has two halves. The follicular phase runs from the first day of bleeding to ovulation and its length varies a lot, both between people and from month to month. The luteal phase, from ovulation to the next period, is much more stable, typically 12 to 16 days with 14 as the standard assumption. That asymmetry is why the reliable way to place ovulation is to count backward from the next expected period, not forward from the last one.
So: next period = first day of last period + cycle length. Ovulation = next period - luteal phase. On a 28-day cycle with a 14-day luteal phase that puts ovulation two weeks after bleeding starts, which is cycle day 15 when the first day of bleeding is counted as day 1 (many sources round this to day 14). On a 32-day cycle it moves to day 19, not day 15, and that is the single most common mistake people make when timing intercourse. On a 24-day cycle it falls on day 11, early enough that the fertile window can overlap the end of the period.
A worked example
Suppose your last period started on March 1, your cycles run 28 days, and your luteal phase is the standard 14 days. Your next period is due March 29. Ovulation is 14 days before that, on March 15, which is cycle day 15 counting March 1 as day 1. The fertile window covers the five days before ovulation plus ovulation day and the day after, so March 10 through March 16. The best two days are March 13 and 14, immediately before the egg is released.
The window is six days wide because sperm survive in fertile cervical mucus for up to five days, while an egg is only viable for 12 to 24 hours after release. Landmark research by Wilcox and colleagues found virtually no pregnancies from intercourse more than five days before ovulation or more than one day after it. If conception happens, the due date is about 266 days after ovulation, which the last column of the table shows.
How much to trust the calendar
Calendar prediction is a starting point, not a measurement. Work using hormone testing has found that only a minority of cycles ovulate on the day a calendar predicts, and that even people with regular cycles see ovulation move by several days from month to month with stress, illness, travel, shift work and weight change. Cycles shorter than 21 days or longer than 35, or that vary by more than about 8 days, are worth discussing with a clinician, as are cycles after stopping hormonal birth control, which often take a few months to settle.
To sharpen the estimate, add a direct signal. Urine ovulation predictor kits detect the luteinizing hormone surge that precedes ovulation by 24 to 36 hours. Cervical mucus turns clear, stretchy and slippery around the fertile days. Basal body temperature rises about 0.5 F after ovulation, which confirms it happened but only in hindsight. Track your actual cycle lengths for three to six months and feed the average into this calculator rather than assuming 28 days. Finally, and importantly, none of this is birth control: the fertile window is wide, it moves, and pregnancies happen on days a calendar calls safe.