To calculate the fertile window, one must first determine the length of their menstrual cycle, which is the number of days from the first day of one period to the first day of the next. Ovulation usually occurs around 14 days before the start of the next period. For a typical 28-day cycle, ovulation would occur around day 14, making the fertile window days 9 to 14. Tools like ovulation predictor kits and mobile apps can help track these days more accurately.
