Guessing how much fondant you need to cover any particular cake is always a bit of a challenge. Most 2 layer cakes are about 4″ high so using a handy chart from Wilton makes it pretty easy.
I have always found that the 5lb box of white fondant that Wilton makes is handy and a good size. Each box contains 2 – 2.5lb packs inside so one pack will be just about right for an average 2 layer round cake.
When doing a tiered cake, simply calculate how much fondant per tier using the chart and then add them all together to get your total amount. Remember, a 5lb box is about 80ozs.