Frosting Calculator
A 9 inch two-layer cake takes roughly 4 to 5 cups. This works out where every one of those cups goes, and hands you the scaled recipe to make it.
How this is calculated
Frosting is a coating, so the honest way to size it is by surface area multiplied by how thick that coat is, not by a lookup table someone copied from another site. Here is every assumption this calculator makes, so you can argue with it.
| Job | Thickness | Applied to |
|---|---|---|
| Crumb coat | 1/20 in | Top and sides. Thin by design: it is glue for crumbs, not a layer of frosting. |
| Filling between layers | Just over 1/5 in | The cut face of each join, so a 3 layer cake has two of them. |
| Outside coat | 1/5 in | Top and sides again, over the set crumb coat. |
| Piped decoration | Percentage of the coat | A simple border adds a quarter. Heavy piping over the whole cake roughly doubles the outside coat. |
Surface area comes from the pan itself. A round cake is π × radius² for the top and π × diameter × height for the sides. A square or rectangular cake is width × length for the top and 2 × (width + length) × height for the sides. Cake height is your layer count multiplied by the layer thickness you chose, because a 4 inch tall cake has twice the side area of a 2 inch one and needs twice the frosting on it. That single detail is missing from most frosting charts, which is why they under-call tall cakes.
Volume is converted to cups at 14.44 cubic inches per US cup, then to grams using a measured weight per cup for the frosting you picked. Those weights come from a real recipe with a real yield, and every ingredient in the scaled list uses the same ingredient weights as the rest of this site, so nothing here can contradict the grams to cups converter.
Where this is a range, not a number. How thickly a person frosts a cake is a preference, and a heavy hand can use half again what a light hand uses on the same tin. Treat the figure as the amount to make, not the amount you will use, and read the honest note below about piping before you commit to a decorating plan.
Read me Piping eats frosting. Spreading does not.
The single biggest reason people run out of frosting is not the size of the cake. It is that they planned for spreading and then decided to pipe.
A spread coat lays down a thin even film. A piped rosette is a solid three dimensional lump of frosting sitting on top of that film, and a border is a continuous rope of it around two edges. Cover a whole cake in piped decoration and you have roughly doubled what the outside coat alone would have taken. Practice swirls and failed attempts get scraped off and add to that, which is why decorators habitually make extra.
The crumb coat pulls in the other direction. It is supposed to be so thin you can still see cake through it. Its job is to trap loose crumbs so they do not drag into your finished coat, and a thick one wastes frosting while making the outside coat harder to smooth, not easier. If your crumb coat looks like frosting, it is too thick.
Practical rule: if you are piping anything beyond a simple border, make an extra half batch and be pleased if it is left over. Frosting keeps for a week in the fridge and most kinds freeze well. Running out with one bare side to go does not keep at all.
Quick reference chart
Total frosting for common pans, with a simple piped border included. Generated by the calculator above, so it can never disagree with it. Screenshot it and keep it with your tins.
| Cake | Cups | Grams | Filling only |
|---|
Grams shown for American buttercream at 247 g per cup. Switch the frosting type in the calculator to see the weight for any of the other seven. Pan sizes assume 2 inch layers. If your recipe was written for a different tin, the pan size converter will tell you how the batter scales, and cake servings will tell you how many people it feeds.
The eight frostings, and when each one is the right answer
| Frosting | g per cup | What it is good and bad at |
|---|---|---|
| American buttercream | 247 g | Butter and powdered sugar. Fastest, sweetest, crusts as it sets so it takes sharp piping. Can feel gritty. |
| Swiss meringue buttercream | 199 g | Silky and much less sweet. Softer, so tall piped shapes slump in a warm room. Cooked whites make it stable. |
| Italian meringue buttercream | 189 g | The most heat stable of the three. Lightest per cup, so a batch goes further. Needs a sugar thermometer. |
| Cream cheese frosting | 235 g | Tangy and soft. Will not hold tall piping and must stay refrigerated. Best spread, not piped. |
| Ganache | 265 g | Chocolate and cream only. Densest of the lot, sets firm, and takes fondant beautifully. Whipping it doubles the volume. |
| Whipped cream | 129 g | Lightest by far and the least sweet. Needs stabilizing, needs refrigeration, and will not survive a warm party. |
| Royal icing | 240 g | Dries rock hard. For cookies, piped detail and gingerbread, not for coating a cake. |
| Rolled fondant | 320 g | Not really a frosting. A sugar sheet rolled and draped, and it needs buttercream or ganache underneath. |
Grams per cup are calculated from each recipe's own total weight divided by its yield, which is why they differ from any single published figure. The meringue buttercreams weigh less per cup than American buttercream because they are half air by the time they are finished, and that is also why they spread further than their weight suggests.
Ganache The three ratios, and what each one is for
Ganache is chocolate and cream in a ratio by weight, never by volume, and the ratio is the recipe. Everything else is technique.
1:1 equal weights. Pourable while warm, and firm enough to spread once it cools. This is the glaze, the drip, and the ganache people mean when they say ganache frosting. Whip it after it has come down to room temperature and it lightens to roughly double the volume, which turns it into a proper frosting. Made with dark chocolate it sets to a soft sliceable set; with milk chocolate it stays looser, so go to about 2:1 for the same body.
2:1 twice the chocolate. Sets firm enough to scoop and roll, so this is the truffle ratio, and it is also the firm filling that holds its line between cut layers. Too stiff to spread straight from the bowl once it is fully set, so bring it to room temperature first if you want to work it.
3:1 three times the chocolate. Very firm. Molded shells, sculpted work, stiff piping and the sharp square edges competition decorators build under fondant. This one sets hard and is not a frosting at all.
Those ratios assume dark chocolate. White chocolate carries far more cocoa butter and much less cocoa solid, so it needs roughly 3:1 to do the job dark chocolate does at 1:1. This is the one substitution that catches people out, and it is why a white chocolate ganache made to a dark chocolate recipe never sets.
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Questions bakers actually ask
How much frosting do I need for a 9 inch two-layer cake?
About 4½ to 5 cups, or roughly 1,150 g of American buttercream, if you want a crumb coat, filling between the layers, a full outside coat and a simple piped border. Drop the border and it is closer to 4 cups. Add heavy piping over the whole surface and it climbs past 6.
Most published charts say 4 to 5 cups without telling you which of those jobs they counted, which is exactly why bakers run out. The breakdown in the calculator above shows where each cup goes, so you can see immediately whether your plan fits the batch you are making.
How much frosting do I need for 12 cupcakes?
About 1½ cups if you are spreading it on with a knife, and about 3 cups if you are piping a tall bakery style swirl. That is a genuine doubling, and it is the most common frosting miscalculation there is.
A knife-spread cupcake takes roughly 2 tablespoons. A piped swirl takes 3 to 4 tablespoons because the swirl is a solid cone of frosting standing above the cake, and a big open star tip makes it bigger still. Set the calculator to cupcakes and match the decoration level to what you actually plan to do.
Do I really need a crumb coat?
For anything with a visible finish, yes, and it is cheaper than you think. A crumb coat is a deliberately thin scraping of frosting that traps loose crumbs against the cake so they cannot drag into your final coat and speckle it. On a 9 inch two-layer cake it costs about 0.6 of a cup, which is a little over one eighth of the total.
Chill it for 20 to 30 minutes until it is firm to the touch before the outside coat goes on. A crumb coat that is still soft simply mixes into the top coat and you have done the work for nothing. You can skip it entirely for a rustic or naked finish, or where the frosting is dark enough to hide crumbs, such as chocolate ganache on a chocolate cake.
How much frosting goes between the layers?
Roughly 1 cup for a 9 inch join, ¾ cup for 8 inch and a scant ½ cup for 6 inch, at a normal filling thickness of just over a fifth of an inch. A three layer cake has two joins, so double it.
Go much thicker than that and the weight of the upper layers squeezes the filling out at the sides, which is what causes the bulge you then have to scrape off and hide. If you want a thick filling, pipe a dam of stiff frosting around the edge first and fill inside it. That is also how you use a loose filling like curd or jam without it escaping.
Is one batch of buttercream enough for a cake?
It depends entirely on the recipe you mean by a batch. The standard American buttercream in this tool, made with 1 cup of butter and 4 cups of powdered sugar, yields about 3 cups. That covers a 6 inch or 8 inch two-layer cake comfortably and falls short on a 9 inch one.
The scaled ingredient list in the calculator solves this by giving you the exact quantities for your cake rather than a multiple of somebody else's batch. If the numbers land awkwardly, round up. Nobody has ever regretted having a spare cup of buttercream in the fridge.
How much fondant do I need to cover a cake?
Roughly 670 g for an 8 inch two-layer cake and 780 g for a 9 inch, rolled to about an eighth of an inch. Published figures for this run anywhere from 600 g to 1 kg for a 9 inch cake, because how much skirt you trim off depends entirely on how neatly you roll, so buy the next packet size up. Set the frosting type to rolled fondant above for your own tin.
Two things to remember. First, you roll a circle far wider than the cake, because it has to reach across the top and down all the sides, and you then trim off a skirt you cannot reuse cleanly; that trim is already included here. Second, fondant is a covering, not a frosting: it needs a layer of buttercream or set ganache under it to stick to and to smooth out the shape of the cake. Pick rolled fondant in the calculator and it drops the crumb coat and filling out of the fondant weight, then tells you separately how much buttercream to make for underneath.
Can I make frosting ahead, and how do I store it?
Yes, and it is one of the few genuinely stress-reducing things you can do before a cake. American buttercream, Swiss and Italian meringue buttercreams all keep about a week refrigerated and up to three months frozen in an airtight container. Ganache keeps two weeks refrigerated.
The critical step is bringing it back. Let it come fully to room temperature, then rewhip it, which takes several minutes and will look curdled and broken partway through. Keep going; it comes back together. Cream cheese frosting and whipped cream are the exceptions: both are best made the day you use them and neither freezes well.
Why does the same cake need more frosting when it is taller?
Because almost all of a cake's surface is its sides, and the sides grow directly with height. A 9 inch cake has 64 square inches of top no matter how tall it is, but a 4 inch tall one has 113 square inches of side and a 6 inch tall one has 170. Adding a third layer therefore adds far more frosting than people expect, and it also adds another filling join.
This is why frosting charts that only list pan diameters mislead you. The calculator above asks for your layer count and layer thickness for exactly this reason, and you can watch the total move as you change them.
Thicknesses are the working figures used in cake decorating practice, not a published standard. Where a figure is a preference rather than a fact, this page says so. Last reviewed August 2026.