Here is how I determined how many bundles of shingles I needed to complete the roof.
First you measure all the surfaces of the roof. The side of my shed is "Side A". From the drip edge to the peak is "Side B" and "Side C" is the back side and in many cases is the same in dimension as the front on a regular shed.
The shed's roof is 18'x16' on the front and the back, or a total of 576^ft.
According to Owens Corning, when someone talks about a square of shingles, they are talking about ruffly 100^ft. Three bundles makes up a square of shingles.
So, a 576^ft roof is going to take 5.76 squares of shingles or 18 bundles when we round up.
Each shingle is 12" wide x 36" long. On the peak, you take the length measurement and multiply that by 2 (18x2=36. On a 18' long peak, it will take you 38 tabs to complete the roof cap. This also includes if you are putting on a ridge vent. How I get 38 tabs is because you want to double the tabs on the ends so the shingles are even.
Hope this information helps. If you have any comments or suggestions please put them in the comment section below.