Calculating weeks pregnant

Need some advice ladies please... for those in early pregnancy, how are you calculating how many weeks you are? Is it from last period or from conception date (if you know) I have 3 different apps with 3 different 'weeks' and lm a bit confused! Medically, l know its calculated from first day of last period so actual weeks is roughly 2 weeks after when ovulated?? I know my period (20th June) and my date of conception (13th July)