I knew this because I knew how much the recipe I used to bake used and how large a bag I needed. Bags actually used to be bigger--which gave some "testing" room.
This reply was modified 4 years, 7 months ago by BakerAunt.
The size of the chips can figure into it, a cup of mini-chips is probably going to be heavier than a cup of standard size chocolate chips (eg, Nestle's Morsels) because there's less air between pieces.
I remember when bags of chocolate chips were 16 ounces, now some of them aren't even 12 ounces. :sigh: