How to use it
Describe the pan the recipe was written for on the left and the pan you want to use on the right. For each pan choose round, square or rectangle, pick centimetres or inches, and enter the diameter, side or length and width. The large number is the multiplier: multiply every ingredient by it. Below it you can paste the recipe’s ingredient list, one item per line, and the tool rewrites each line with the first number scaled, so “200 g all-purpose flour” becomes about 253 g when you move from an 8-inch to a 9-inch round pan. Lines with no number, such as “a pinch of salt”, are kept and flagged so you can decide for yourself.
How the conversion works
The multiplier is the area of your pan’s base divided by the area of the recipe’s pan. A round pan’s area is π times the radius squared; a square is the side squared; a rectangle is length times width. Inches are converted at exactly 2.54 cm, the international definition given in NIST Special Publication 1038, so both pans are compared in square centimetres whatever units you typed. Because area grows with the square of the diameter, small changes add up fast: going from a 6-inch to an 8-inch round pan needs about 1.78 times the batter, not 1.33.
The method assumes both pans are filled to the same depth, which keeps the cake the same height and the texture close to the original. Mixing shapes is fine. An 8-inch square and a 9-inch round are nearly equal, while a 9 by 13-inch rectangle holds about 1.83 times an 8-inch square.
When to double-check
Eggs are the usual problem. Scaling 3 eggs by 1.27 gives 3.8, which you cannot crack. Round to 4 and adjust the liquid a little, or beat the eggs and weigh them, at roughly the proportion the multiplier asks for.
Pan depth and baking time are outside the calculation. If your pan is much deeper or shallower than the recipe’s, fill it to a similar level and keep leftover batter for cupcakes. Check doneness earlier than the recipe says when the cake is thinner, and later when it is thicker. If you are unsure of a pan’s size, measure the inside across the top with a ruler; the rim and sloped walls can make it look bigger than the batter area really is.