Welcome to the Cake Decorators Q&A

asked July 5th 2012


hi , might sound a daft question but here goes. If I have say a 6inch recipe , how  do I work out the quantaties for say an 8inch tin, 10inch tin and so on .


Hi dinky, last post was lost in space, here is the mathematical formula. This is my hubby’s conversion for me. He is a scientist so has had to use all sorts of formulae in his work.

How to convert a cake recipe from one tin size to another

Mathematically, it is simply a matter of applying the surface area ratio to each ingredient.
1/ Calculate the surface area of the target tin. Then divide this by the surface area of recipe tin. This will give you your conversion factor.
2/ Multiply each of your ingredients by this factor to get the required quantities.
To calculate the surface area of a square tin, multiply the size of the tin by itself ( if rectangular multiply length by the breadth )
To calculate the surface area of a round tin, half the diameter of the tin. This is called the radius.
Multiply the radius by itself and multiply this figure by 3. Three is an approximation of a mathematical constant called Pi (More accurately it is 3.1416. But 3 is close enough.) You may remember the equation from your GCSE maths : Pi x R squared = the area of a circle. This is it.
Example 1. Convert the recipe for an 8 inch round cake to a 10 inch square.
Surface area of 8 inch round:
Radius ( 8 divided by 2) = 4. Radius squared (4 x 4) = 16.
16 x Pi (3) = 48
Surface area of 10 inch square (10 x 10) = 100
Factor to use: Surface area of target tin 100 divided by surface area of recipe tin 48.
This figure is very close to 2.
In this case simply double the quantities.
Example 2. Convert recipe for a 12 inch round to a 7 inch square.
Surface area of 7 inch square ( 7 x 7 ) = 49 (Target)
Surface area of a 12 inch round:
Radius squared (Half of 12 x Half of 12 or 6 x 6) = 36 x 3 (Pi) = 108. (Recipe)
Factor required: Target surface area (49) divided by Recipe surface area (108) = One half approximately.
In this case halve all the ingredients.
If the result I get gives me a fraction of an egg, I usually round the recipe up to get whole numbers of eggs. This gives a deeper cake, which can then be trimmed if necessary.
You cannot use this factor to estimate the baking time. You need to use your own experience with the mixture and your oven.
I hope this helps you and other members who have asked similar a question. I’d love to know if it’s been of help. xx

There must be a science behind it so happy for anyone to add something more constructive but here goes….

Just through trial and error we’ve worked out just through basing everything on how many 10″ cakes we get from a mix and doing some very rough maths from that. Strangely though it never seems to work exactly the same every time – now that is one for the scientists.


Google lindy smiths conversion chart (it’s on her website). She explains how to multiply to work out for different sizes of tins


hmmm. thanks david  thought it might be trial and error.

and thanks jmilne I will have a look at lindy smith


Hi dinky,  I posted the mathematical formula on here last night but for some reason it hasn’t appeared. Using wifi at the moment, may be that is why it is delayed. Keep an eye out for it, this is not the first time it has happened. Will post again if it has been lost in space!


thanks madeitwithlove, not got it yet



hi madeitwithlove, sorry not replied earlier. my coputer broke. I found this hard to follow. I have gone with the lindy smith conversion chart.  thanks anyway


Thanks, madeitwithlove! I’ll be far more confident attempting conversions knowing why I’m using a particular conversion factor. If that doesn’t explain how much I appreciate your formula post, I’ll try a simpler way: Thank you, thank you, thank you!!! *Hugs* .^_^.


Hello darlinghuskey

You can also easily make conversions here http://www.cakeflix.com/blog/how-to-work-out-what-size-cake-tin-to-use
Both the ready reckoner or the the cakeometre are simple tools to use but as you say knowing the formula does give confidence. x

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