First of all woman has to know how long is her menstrual cycle. Statistically, average female has a cycle 28 days long, but basically anything from 20 to 40 days is still considered normal. Length of menstrual cycle is considered from the first day the menstruation started to the day before the menstruation in the next cycle starts. But if woman has irregular periods, this method is not the most appropriate. Second important data is length of luteal phase, which is calculated in number of days.
Luteal phase is also referred as days past ovulation and is actually that part of the menstrual cycle that starts after the ovulation day and ends one day before your next menstruation starts. Lutheal phase actually represents that time of the month when new egg starts to develop. Statistically, average woman s lutheal phase is 14 days long, but anything between 10 and 20 days is still considered normal for healthy egg maturation. Calculating ovulation date is pretty easy, and all you have to do, is to apply following formula:
Number of days of menstrual cycle - Number of days of luteal phase = Ovulation day
For example, if you have a menstrual cycle that 30 days long (30 days from your first day of menstruation to the one day before the next cycle begins), and if you luteal phase is 14 days long (days past ovulation until the next cycle starts), then your most fertile day will be on the 16th day since the beginning of your menstrual cycle. Third data that is useful is the exact date when your menstruation started add 16 days (from the calculation above) and you get the date of your ovulation.
This method is not the most precise, but nevertheless works very well for women who have regular periods and of course for women that do not have any medical conditions that may affect their ability to conceive. For more accurate and precise technique I recommend ovulation kits that measure luteinizing-hormone.