Welcome to the Cake Decorators Q&A
How do you use the cake recipe calculator
To use the cake re-recipe calculator at... https://www.designer-cakes.com/cake-size-calculator
The top drop down menu asks whether you want to choose your own recipe or one that's already pre-programmed from the online tutorial on the site.
If using your own recipe then you need to enter the ingredients and quantities used. You then select the size of cake the recipe was designed for then below select the destination size (the size you are looking to bake). The calculator will then adjust the quantities to meet the size of the target cake.
If you are using on of the tutorial recipes then its just a case of entering the size and shape of the cake you are looking to bake and the calculator will work out the ingredients and quantities needed.
Hope this helps, but please let me know if you have any questions at all.