Buttercream Icing Calculator

How to use this calculator

1. Measure the finished cake, not the tinHeight means the assembled height including the filling, since that is what the sides have to cover.
2. Count layers, not spongesThree sponges give two filled layers. The calculator asks for layers and works out the fillings from that.
3. Pick the finish honestlyA crumb coat uses a third of a full cover. A thick pipeable finish uses half again as much. It is the biggest single variable.
4. Make a little more than the answerTen to twenty per cent spare is sensible. Running out mid-cover is genuinely hard to recover from.

Area, not volume

The instinct is to scale buttercream with the size of the cake. That is nearly right and it is wrong in a way that matters, because buttercream covers a surface rather than filling a space.

Surface area scales with the square of the diameter. Volume scales with the cube. So a cake twice the diameter has four times the top area and eight times the volume, and needs four times the icing rather than eight or two.

For a round cake the top is πr² and the side is π × diameter × height. Those two together are what a full cover has to reach.

The side is often the larger of the two on a tall cake, which surprises people who have only thought about the top. A 20 cm round cake 10 cm tall has a 314 cm² top and a 628 cm² side — the sides need twice as much as the top.

That is why tall layer cakes use so much more buttercream than a traditional shallow sandwich cake of the same diameter.

Filling is a separate calculation. Each filled layer is another circle of buttercream at roughly the area of the top, so a three-sponge cake has two fillings and needs two extra circles' worth.

The coverage rate depends entirely on the finish. A crumb coat is a thin sealing layer that traps loose crumbs before the final coat, and it uses very little. A semi-naked finish is scraped back so the sponge shows through. A full smooth cover needs enough thickness to hide the cake and be scraped level. A thick finish for piping and detail needs more again.

Those four differ by more than four to one, which is why any single figure for how much a cake needs is meaningless without knowing what it is being used for.

Cupcakes work differently. A piped swirl is a fixed quantity regardless of the cake underneath — around 45 grams for a generous swirl, or 25 grams spread flat with a palette knife. Twelve cupcakes with swirls need about the same buttercream as a small layer cake.

Worked example: a 20 cm round, two layers, full cover

A 20 cm round cake, 10 cm tall, with two layers and a full smooth cover.

The top is π × 10² = 314.2 cm².

The side is π × 20 × 10 = 628.3 cm².

Together that is 942.5 cm² of outside surface.

At 30 grams per 100 cm² for a full cover, the outside needs 282.7 grams.

Two layers means one filling, which is another 314.2 cm² at about 22 grams per 100 cm² — 69.1 grams.

Total: about 352 grams of buttercream.

At the classic 1:2 ratio that splits into roughly 117 grams of butter and 235 grams of icing sugar, plus a little milk to loosen and a teaspoon of vanilla.

Now change the finish. The same cake with a crumb coat only needs 163 grams. Semi-naked needs 239. A thick pipeable finish needs 493.

That is a three-fold spread on an identical cake, entirely from the finish.

Now change the size. A 25 cm round of the same height has a top of 490.9 cm² and a side of 785.4 cm² — 1,276 cm² total, needing 383 grams for the outside and 491 grams including the filling. The diameter went up 25% and the total went up 40%.

And a 30 cm round needs 650 grams in total — 85% more than the 20 cm cake for a 50% larger diameter.

Now the ratio. Three hundred and fifty-two grams of buttercream at 1:1.5 gives 141 g butter and 211 g sugar — noticeably less sweet and softer. At 1:2.5 it gives 101 g butter and 251 g sugar, which is firmer and holds a piped edge better.

The total quantity does not change with the ratio; only what it is made from does.

Finally, cupcakes. Twelve with a generous piped swirl need 12 × 45 = 540 grams — more than a fully covered 25 cm layer cake. Piping uses far more than people expect, which is why a batch of cupcakes so often runs short.

Buttercream by cake size and finish

Two-layer cakes with one filling included, taken from the calculator above. Note how quickly the numbers climb with diameter — a 30 cm cake needs nearly three times the 15 cm one, because area scales with the square of the diameter.

Note too that twelve piped cupcakes use more than a 30 cm layer cake at semi-naked finish, which is the figure most people get wrong.

Making it, colouring it, and the things that go wrong

Beat the butter alone before adding any sugar, and beat it for longer than feels necessary — three to five minutes with an electric mixer. It goes visibly paler as air is worked in, and that aeration is the difference between light buttercream and a dense greasy one.

The butter must be properly softened but not melted or oily. Straight from the fridge it will not aerate; too warm and the mixture splits. Room temperature in a cool kitchen, which means taking it out an hour or two beforehand rather than microwaving it.

Sift the icing sugar. Lumps do not beat out and grainy buttercream cannot be rescued — the only fix is to start again.

Add the sugar in stages with the mixer slow, or the kitchen disappears in a cloud. Then beat properly once it is all in.

A pinch of salt cuts the sweetness noticeably and is worth adding even though it is not traditional. A tablespoon or two of milk or cream loosens the mixture to a spreadable consistency.

On ratios: 1:2 butter to icing sugar is the classic British proportion and is quite sweet. 1:1.5 is less sweet and softer — better for eating, harder for detailed work. 1:2.5 is firm and holds a piped edge cleanly, which matters for detail and in a warm kitchen.

Swiss and Italian meringue buttercreams are less sweet again and considerably more stable, but they are a different method involving cooked egg white and are outside what this calculates.

For colouring, use gel rather than liquid. Liquid colour thins the mixture and can split it, and it takes a great deal to reach a deep shade. Colours darken as they stand, so mix a shade lighter than you want and leave it an hour before judging.

Red and black are notoriously difficult and need far more colour than expected. Starting from a chocolate buttercream makes black much easier to reach.

On crumb coating: it is not optional for a smooth finish. A thin layer traps loose crumbs, chilled for twenty minutes, then the final coat goes on clean. Skipping it means crumbs in the finish and no way to remove them.

A cake iced with buttercream keeps two to three days at room temperature in a cool place. Refrigerating firms it hard and dries the sponge, so bring it back to room temperature well before serving if you must chill it.

Finally, make extra. The single most common problem is running short with a quarter of the cake left to cover, and a fresh batch mixed separately rarely matches the first for colour or consistency.

Assumptions and limitations

• Coverage rates are practical figures rather than precise measurements
• Assumes an American-style buttercream of butter and icing sugar
• Does not cover Swiss, Italian or French meringue buttercreams
• Assumes even layers of the same diameter
• Does not allow for piped decoration beyond the finish selected
• Cupcake figures assume a standard case and a generous swirl
• Does not account for a covering of fondant over the buttercream

Questions people ask about this in United Kingdom