The Mathematics of Fondant Draping
Calculating fondant requirements is an exercise in 3D surface area estimation. Fondant is essentially a sugar-based fabric; you need enough to cover the top of the cake plus the entire vertical 'skirt' of the sides. Because you must drape the fondant and smooth it downward to prevent pleating, you must always roll the fondant significantly larger than the theoretical surface area.
Calculating the Target Diameter
To cover a cake, you must roll the fondant into a circle with a diameter equal to the Cake Diameter + (2 x Cake Height) + 2 inches of buffer.
The Weight-to-Area Ratio
Professional fondant is typically rolled to a thickness of 1/8 inch (3mm). At this thickness, you can estimate the required weight based on the total square inches of the cake's surface.
Weight (oz) = [Area of Top + Area of Sides] * 0.12
Accounting for Waste
The 'Trim' or 'Scrap' is a necessary part of the process. You cannot cover a cake with the exact amount of fondant needed; the weight of the excess hanging off the sides is what helps pull the fondant flat against the cake. This scrap can be gathered, cleaned of crumbs, and reused, but you must have it on hand for the initial application.