Fondant Coverage Calculator

To cover a cake you have to roll out a single flat sheet big enough to drape over the top and all the way down the sides — a disc whose radius is at least the cake radius plus its height, before any trimming. That rolled sheet is always larger than the cake's own surface, so the calculator sizes the weight from the sheet you actually roll, not just the cake surface. Rolled fondant is essentially solid sugar paste with a density of about 1.35 g/cm³, so a layer 5mm thick weighs roughly 0.675g per cm² of sheet.

The Formula

• Round cake surface area: pi x r² (top) + 2 x pi x r x height (sides)
• Square cake surface area: side² (top) + 4 x side x height (sides)
• Round roll diameter: cake diameter + 2 x height + trimming allowance
• Square roll diameter: diagonal (side x 1.41) + 2 x height + trimming allowance
• Fondant weight: rolled-sheet area (pi x (roll diameter / 2)²) x 1.35 g/cm³ x thickness (cm) — about 0.675g per cm² at 5mm

Charts published by UK sugarpaste suppliers tend to quote slightly higher figures because they build in extra for trimming and drape, so buy about 10-15% more than the calculated weight.