Safe period is calculated by -

Safe Period (rhythm method) This is also known as the calendar method first described by Ogino. The method is based upon the fact that ovulation occurs from 12 to 16 days before the onset of menstruation. Calculation is as follows : The shortest cycle minus 18 days gives the first day of the fertile period. The longest cycle minus 10 days gives the last day of fertile period. For example, if a woman's menstrual cycle varies from 26-31 days, the fertile period during which she should not have intercourse would be from the 8th day to 21st day of the menstrual cycle, counting day one as the first day of the menstrual period.