The rhythm method

Please don't EVER try to use this as a contraceptive method. It is definitely NOT one. The ovulation happens about two weeks after the last period (and the sperm can "wait" for the egg for about 7 days, too), so according to your calculation you assume to be infertile when you're actually extremely fertile.
If you don't wanna use a barrier method, get some more information about NFP. It's not too complicated, you just need to be disciplined. If you do it correctly, it's as safe as a condom .

 

you should know your specific cycle so you won't get pregnant by chance

 

Just use frickin birth control...jeez.