How do you calculate your EDD?

Question

Hey mamas! 
I was just wondering if anyone could help me out, since I’m in Canada my first prenatal appointment is at the 12 week mark. 
How do you guys calculate your EED?
The start of my last period October 10th I put this into a random app and it says July 16th so I wanted to see how reliable it was, or what you ladies do.
I’m excited but it’s a long wait for my first ultrasound 😂
TIA!

Answer

The start of my last period was 10/11 but glow is telling me I’m due July 12?

Answer

The start of my last period was also 10/10 and right now it’s showing 7/16 as well! I have my first appointment after 8 weeks (on December 8th) so I’m hoping to get a more accurate due date during my first ultrasound, but it shouldnt be a drastic difference.