At the end ofthe day it comes down to some simple maths. Here's a close up of a garter stitch scarf:
Looks a bit like a brick wall, doesn't it. Each stitch is like one brick. It took three 50 g balls to make this scarf. What if I want a bigger or smaller one? To work out how much wool I need I just need to know how many bricks there are and how much wool I need on average for each one . . . done!