How many weeks?

Courtney • Hello I'm Courtney, im 27 and am expecting my first child with my fiance Eric. Praying for a smooth pregnancy and a healthy baby! Look forward to interacting with you all! :-)
I had my last cycle 09/25 so I count 6 weeks today but yesterday, my ultrasound said I was 5 weeks 1 day. The nurse told me that the ultrasound measurement is 100% accurate. I'm completely confused and when I asked, she didn't really explain how they got to that calculation. Does any one know which one to go by to determine due date?