I have rechecked this 3 times but now I am at the point where I have looked at it so much it's starting to look wrong KWIM
I have a 6-8-10-12" round wedding cake
8-12-16" round wedding cake coming up
I have calculated (using the Wilton chart) that I will need 386 oz of fondant, which comes out to just over 24 lbs. So let's round up to 25 lbs.
Does that sound right? I will probably order 30 lbs just to be safe
I don't measure icing or fondant by weight or cups, but I would suggest that you make a note of how much fondant you need/use for each cake, then you'll have a reference for future use.
That's how I determined that I need 4 2-lb bags of p.sugar for every 7 cake mixes and could have some icing left over. That's my "measure"!