Running out of frosting halfway through decorating is frustrating, and making far too much wastes butter and sugar. This calculator estimates the buttercream needed to fill between the layers and coat the top and sides of a round or square cake, with extra for piping borders and decorations.
How the calculation works
- Top = area of the top × coating thickness
- Sides = perimeter × height × coating thickness
- Filling = area × filling thickness × (layers − 1)
- Total = top + sides + filling, plus extra for decoration
One US cup holds about 14.4 cubic inches (237 mL).
Example
An 8 in round cake with two layers, 4 in tall, coated and filled with 1/4 in of buttercream needs about 3.5 cups: 0.9 cup for the top, 1.7 cups for the sides and 0.9 cup for the filling. With 20% extra for piping, plan about 4.2 cups.
Typical amounts (2 layers, 4 in tall)
| Cake | Fill and coat | With 20% for piping |
|---|---|---|
| 6 in round | about 2.3 cups | about 2.7 cups |
| 8 in round | about 3.5 cups | about 4.2 cups |
| 9 in round | about 4.2 cups | about 5 cups |
| 10 in round | about 4.9 cups | about 5.9 cups |
| 9 × 9 in square | about 5.3 cups | about 6.4 cups |
How much is a batch?
A standard American buttercream made with 1 cup (2 sticks) of butter and 4 cups of powdered sugar gives roughly 3 to 3.5 cups. Swiss and Italian meringue buttercreams are lighter and give more volume for the same weight.
Tips
- Apply a thin crumb coat first, chill the cake for 20 to 30 minutes, then add the final coat.
- A thicker filling (1/2 in) or a dam of stiff buttercream around a soft filling such as jam or curd needs more frosting.
- Drip cakes, rosettes and heavy piping can easily need 50% extra.
- Leftover buttercream keeps a week in the fridge or three months in the freezer; bring it to room temperature and rewhip before using.