It uses a whole can of soda per serving Sheryl.
A&W Regular 12 oz can cream soda - 120
A&W Diet 12 oz can cream soda - 0
So if you decided to have two, you could knock of 240 calories.
BTW, per serving made regular, this drink is, I believe I read 500 calories per serving.
So two wit regular soda, 1000 calories, Two with diet soda, 760 calories.
It all adds up at the end =)