Confused

I know you don’t get ur actually due date till you have your scan but I’m wanted to book a reassurance scan and need to know how many weeks I am my apps and the calculator on line tell me all different it ranges from 5 weeks 2 days to 6 weeks ? Can someone help me the first day of my last period was the 1st of December and I usually have a 30 day cycle how many weeks am I? Thankyou