How due i calculate my EDD?

Sarah • 30, SO and I have 8 1/2yr old boy, 6yr old girl and 4yr old boy and expecting another baby 🥰
My AF was Feb. 16-20 and I got my positive test march 21 and 22nd. How far along does that make me? It says I'm 5 weeks but I don't understand how that's possible cause that's when I got off my AF? I need help understanding this?