How many weeks pregnant?

Emma
Hello everybody!
I am just a little bit confused because this app tells me I am 5 weeks and 4 days pregnant. The What to Exclet app tell me 4 weeks and 4 days...! Pretty different! One tells me my due date is december 23td, the other says December 27th! 
Do you know if this is because there are different ways to calculate this? 
When you say "I am 5 weeks pregnant" for example, do you count the weeks starting at your first day of last period or day of conception?
I think 4 weeks and 4 days is more accurate for me as my last period started March 22nd. 
Also, when you all talk about your HCG levels, do you know them because you are doing blood tests or are there tests you can do at home to know those?
Thank you!!!