Calculating pregnancy with irregular period

Patient: I haven’t had my period since December and Haven’t had it since and it’s now January but I had un protected sex two weeks ago and I just want to know how many weeks can I be pregnant because I have been reading that I can calculate how many weeks I’m pregnant since my last menstrual cycle. But due to my irregular period I’m just a little confuse. Because I hadn’t had sex until two weeks ago. And I took a pregnancy test and it came out positive so o I’m pregnant just need help on calculating how far i am