The general rule to remember regarding how to convert the recipe between round and square cake tins is:
Use a Round Cake Tin 1 size LARGER than a Square Cake Tin
For example:
The recipe quantities for a 6 inch Square Cake Tin are the same as you would use for a 7 inch Round Cake Tin.
The recipe quantities for an 8 inch Square Cake Tin are the same as you would use for a 9 inch Round Cake Tin.
From a practical point of view, that’s all you need to know.
How the Rule is Calculated
For those who wish to understand how the rule is obtained, you need to consider the following.
1) Area of Square Cake Tin = length(l) x width(w)
Given the length and width of a square are the same, we can rewrite it as l2
2) Area of Round Cake Tin = Pi(3.14) x radius squared (radius is half of the tins diameter)
We can rewrite it as 3.14r2
Square Cake Tin to Round Cake Tin
Example: 6 inch Square Cake Tin
Area of a 6 inch Square Cake Tin = l2 = 6 x 6 = 36 square inches
Area of circle = 3.14r2 which can be rearranged as: r2 = Area of circle ÷ 3.14
For comparision purposes, if we use the area value for a 6 inch Square Cake Tin i.e. 36, then:
r2 = 36 ÷ 3.14 = 11.46
Therefore, r = √11.46 = 3.39
The diameter of a Round Cake Tin is 2r, which equals 2 x 3.39 = 6.78
6.78 approximates to a 7 inch Round Cake Tin
Round Cake Tin to Square Cake Tin
Example: 7 inch Round Cake Tin
The diameter of a Round Cake Tin is 2r, therefore the radius is 7 ÷ 2 = 3.5
Area of circle = 3.14r2 = 3.14 x (3.52) = 38.47 square inches
Area of a square = l2, which can be rearranged as: l = √area of square
For comparision purposes, if we use the area value for a 7 inch Round Cake Tin i.e. 38.47, then:
l = √38.47 = 6.2
6.2 approximates to a 6 inch Square Cake Tin
Summary
As a rough but useful guide, remember to:
Use a Round Cake Tin 1 size LARGER than a Square Cake Tin