Thanks for the query.
What you are talking about is Rhythm method/ Calender method of Contraception
First of all, to practice this method your menstrual cycles should be regular for atleast 3 months. You need to confirm on that.
What is the duration of your menstrual cycle? I mean to say, once in how many days to you have periods? For eg: once in 30 days or once in 25 days, etc.
However, it is always advisable to use Barrier method of contraception ( condoms ), as it not only prevents you from getting pregnant but also protects you from contracting HIV
and other sexually transmitted diseases.