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

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).

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.