The most accurate way to calculate ovulation day is to push forward 14 days from the first day of menstruation, this time is relatively fixed, before and after the difference of 2 days, 12-16 days before the onset of menstruation, will be ovulated during this time, each person’s menstrual cycle is different, some people also irregular menstruation, but after ovulation menstruation time is relatively fixed. If the menstrual cycle is usually prolonged or shortened, it is caused by the prolongation or shortening of the follicular phase, some people will have their period in 21 days, some people will have their period in 35 days, this is the normal range. If you get your period on 21 days, ovulation should be on the 7th day of your period. It is more accurate to calculate the ovulation date according to this rule.