This application calculates the days for a woman to avoid sex as a natural method of birth control.
Note that there are no actual safe days you can calculate with this equation. Menstrual cycles can change month-to-month and there is no guarantee this method will prevent pregnancy. Using this equation will increase your chance of not getting pregnant but will not ensure that you will not get pregnant. If you don't want to get pregnant for sure, use birth control or abstain.
This family planning method has been developed as an algorithm which is meant to identify which days of a month one's chances of not conceiving are higher.
This is mainly aimed at explaining a traditional and historic family planning method that has repeatedly failed for many couples.
An example of a menstrual cycle might have 28 day cycle between the start of a woman's periods.
See the following link for a similar representation of a calendar method: Calendar method