Fertility calendar method??

Does anyone use this method and how do you like it ? I am interested in it, how it works and how effective it is . I'm still leaning how this works so if you have any experience I would appreciate your output . My husband and I had unprotected sex on my cycle day 6 and I didn't take anything, I'm not on my firtle days but he did finish inside . How big/small are the chances of being pregnant ? Thanks in advance ??