How to scale a cake recipe up or down

How to scale a cake recipe up or down

 This post will help you work out exactly how to scale up (or down) any cake recipe so that you make the perfect size cake for you! 

Square cake tins versus round cake tins

I get lots of comments asking me how to adjust my cake recipes for different sized cake tins. But the first question I ask is what type of tin the reader is using (e.g. is it square or round). You might wonder why this matters – aren’t all 8 inch cake tins the same?! The short answer is no. A square tin actually holds roughly 25% more cake batter than a round tin of the same size!

This means that if the recipe calls for a square cake tin but you want to use a round tin, you will need to size up. Don’t worry, the conversion chart below covers this for you. 

Conversion chart for my Christmas Cake


These cakes cook best low and slow. So regardless of what cake tin you are using, these cakes should be baked at 135C / 275F / gas mark 1.

Ingredients For a 6 inch (15 cm) round or a 5 inch (12 cm) square tin For a 7 inch (17 cm) round or 6 inch (15 cm) square tin For an 8 inch (20 cm) round or 7 inch (17 cm) square tin For a 9 inch (22 cm) round or 8 inch (20 cm) square tin
Dark brown sugar 150 grams 200 grams 250 grams 300 grams
Butter 150 grams 200 grams 250 grams 300 grams
Eggs 3 4 5 6
Plain flour 250 grams 300 grams 350 grams 400 grams
Sultanas 175 grams 240 grams 320 grams 400 grams
Currants 130 grams 180 grams 250 grams 300 grams
Raisins 45 grams 60 grams 75 grams 100 grams
Mixed peel 70 grams 90 grams 120 grams 150 grams
Glace cherries 45 grams 60 grams 75 grams 100 grams
Chopped almonds 45 grams 60 grams 75 grams 100 grams
Cinnamon 0.25 tsp 0.5 tsp 0.75 tsp 1 tsp
Dark treacle 1 tbsp 1.25 tbsp 1.5 tbsp 2 tbsp
Brandy 130 millilitres 180 millilitres 250 millilitres 300 millilitres
Approx. cooking time 3.5 hours 4 hours 4 hours and 30 minutes 4 hours and 45 mins

Different sizes of fruit Christmas cake

The maths

This conversion chart references the ingredients in my traditional Christmas cake and my naked Christmas cake glazed with nuts and cherries. But, with the following equation you can convert any cake recipe.

To scale down any cake recipe

In this following formula:

  • a = the diameter of the cake tin you are using
  • b = the diameter of the cake tin the recipe calls for

(a x a) / (b x b) = the factor by which you need to multiply the ingredients in the original recipe

For example:

I have a 6 inch tin (this is “a”) but the recipe calls for an 8 inch tin (this is “b”). If I plug these numbers into the formula I get the following:

(6 x 6) / (8 x 8) = 0.5625

To get the new ingredients list I must multiply each ingredients by 0.5625.

For example:

100 grams of sugar becomes 56.25 grams and 150 grams of butter becomes 84.375 grams.

These are very specific values and you should be able to round up to the nearest gram with little affect on the overall recipe.

To scale up any cake recipe

We can use the same formula to scale up a cake recipe.

As a reminder, in this following formula:

  • a = the diameter of the cake tin you are using
  • b = the diameter of the cake tin the recipe calls for

(a x a) / (b x b) = the factor by which you need to multiply the ingredients in the original recipe

For example:

I have a 9 inch tin (this is “a”) but the recipe calls for a 7 inch tin (this is “b”). If I plug these numbers into the formula I get the following:

(9 x 9) / (7 x 7) = 1.653

To get the new ingredients list I must multiply each ingredients by 1.653.

For example:

3 eggs becomes 5 eggs and 200 grams of flour becomes 330 grams of flour.

Again, you should be able to round up to the nearest gram with little affect on the overall recipe. If you find that you need to round up the number of eggs then always be sure to make sure you have also rounded up your butter, sugar and flour (rather than rounded down).

Different sized fruit cakes - 3 tiers

How do you convert cooking times?

I wish I had a handy equation for this question, I really do.

Because my Christmas cake is cooked low or slow I just adjust the cooking time (a smaller cake quite simply takes less time to cook!)

If you are adjusting the size of your cake tin by more than 1 inch I would recommend altering the heat too.

All you can do then is check on the cake regularly to see if it is springy to the touch, pulling away from the edges of the tin and a skewer poked in the centre comes out clean. These are all good indications the cake is ready.

Good luck.



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