What is the ratio for buttercream?

Classic British buttercream uses a 2:1 ratio of icing sugar to butter by weight. So for every 100g of butter, you need 200g of icing sugar. Add a small amount of milk (about 1 tablespoon per 250g icing sugar) to reach spreading consistency.

How much buttercream do I need for a 20cm cake?

For a 20cm (8 inch) round cake with filling and coating, you need approximately 500g of buttercream total — about 150g for the filling between two layers and 350g for the crumb coat and final coat on the outside.

Should the butter be soft?

Yes, the butter should be at room temperature (about 20°C) and soft enough to beat easily but not melted or greasy. Cold butter will create a lumpy buttercream, and melted butter will make it too runny. Beat the butter alone first until pale and fluffy before adding the icing sugar.

How do I make chocolate buttercream?

Add 2-3 tablespoons of cocoa powder per 250g icing sugar, or fold in 100g of melted and cooled dark chocolate per 500g of finished buttercream. Reduce the milk slightly when using melted chocolate as it adds moisture.

Can I make buttercream ahead of time?

Yes, buttercream keeps well in an airtight container in the fridge for up to a week, or frozen for up to 3 months. Bring to room temperature and re-beat before using to restore its fluffy texture.

Why is my buttercream grainy?

Grainy buttercream is usually caused by under-beating or using icing sugar that is not fine enough. Beat the buttercream on medium-high speed for at least 3-5 minutes after adding all the icing sugar. Using a stand mixer gives the best results.

BakeCalcs

Buttercream Calculator

Calculate butter, icing sugar, and milk quantities for buttercream based on cake size, layers, and coverage type.

Cake Size (Diameter)

Number of Layers

Coverage TypeChoose whether to estimate buttercream for the filling between layers, the outer coating (crumb coat + final coat), or both.

Adding flavouring (vanilla, cocoa, etc.)

Flavourings add minimal weight. Turn this on when using liquid extracts (vanilla, etc.) and the milk figure is trimmed slightly to keep the consistency right.

How We Calculate This

This calculator estimates the total buttercream needed based on your cake size, number of layers, and whether you want filling, coating, or both. It then breaks this down into ingredient quantities using the classic 2:1 icing sugar to butter ratio.

The Formula

Filling per layer: ~150g for a 20cm cake, scaled by area ratio (size/20)²
Coating (crumb + final): ~350g for a 20cm cake, scaled by area ratio
Icing sugar: 2/3 of total buttercream weight
Butter: 1/3 of total buttercream weight
Milk: ~15ml per 250g icing sugar (trimmed by ~12% when you tick the liquid-flavouring option, as extracts add moisture)

Frequently Asked Questions

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Last updated: February 2026

All calculations are based on standard ratios and conversions. Results may vary with specific ingredients, equipment, and conditions.