Frosting Quantity Calculator
Work out how much buttercream a cake actually needs before you start mixing. Enter the size, layers, and height, and get a breakdown for filling, crumb coat, finish, and decoration — in cups and grams.
Total height is the finished stacked height of the cake, used for the sides.
Coat thickness & frosting
Coat thicknesses and density are defaults you can adjust. Whipped or light buttercreams weigh less per cup — around 0.8–0.9 g/mL — so lower the density for those.
Formula & how it works
Filling volume is top area × fill thickness × (layers − 1). Each coat is (top area + side area) × coat thickness, where side area is perimeter × height.
Decoration adds a percentage of the running total, and everything is converted to cups and grams using the frosting density you set.
Worked example
An 8-inch round cake, three layers, 4 inches tall: the top area is 50 in² and the sides about 100 in². Two fillings at a quarter inch add 25 in³, a thin crumb coat about 8 in³, and a finish coat about 23 in³. With a 15% decoration allowance that totals roughly 64 in³ — around 4.4 cups, or a little over 1 kg of buttercream at 1 g/mL.
Planning your batch
Fillings scale with layers, coats with surface
The more layers you stack, the more filling you spread — a three-layer cake has two fillings, a four-layer has three. The crumb and finish coats depend instead on the outside surface, which grows with both diameter and height. Splitting the estimate this way means a tall, narrow cake and a short, wide one of the same volume can need quite different amounts.
Decoration is where estimates drift
A smooth finish uses close to the calculated amount, but heavy piping, rosettes, or a thick shell border can add far more than the default allowance. If you have a decorating plan in mind, raise the decoration percentage; a plain semi-naked cake can drop it toward zero.
Weigh your frosting for repeatable results
Cups of buttercream vary with how much air is whipped in, so the gram figure here is the more reliable target if you weigh as you make it. Lighter, fluffier frostings weigh less per cup — lower the density input for Swiss meringue or whipped styles, and keep it near 1 for a dense American buttercream.