Figuring out estimated due date

Jesmarie • 19 years old with rainbow baby on the way! 9/30/2017 ❤️
I've recently found out I am expecting and wanting to know my due date.. I've heard you're supposed to go by the first day of your last period and since I was on birth control it would make my period come one month and miss the next. So how would I be able to calculate my estimated due date if I got my period in Dec but not Jan?