3 ways. 1) look at a baking pan chart, for example: http://dish.allrecipes.com/cake-pan-size-conversions/ see that the 9x13" pan (usually 2" high) holds 16 cups. Fill the 12x17 with water and you have a cup measure. Divide that by 16. Result is how much of original recipe to make. 2) Do the math. Multiply the length x width x height" (usually 2") for each. That gives you cubic inches. Divide the cu in of the 12 x 17 over that of the 9 x 13 and you get your recipe factor to make. 3) Look at this Alice Medrich article about making cakes fit the pans you have. Your pan size might be in there. https://food52.com/blog/13239-how-to-make-your-baking-recipe-fit-your-pan-size
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1) look at a baking pan chart, for example:
http://dish.allrecipes.com/cake-pan-size-conversions/
see that the 9x13" pan (usually 2" high) holds 16 cups.
Fill the 12x17 with water and you have a cup measure. Divide that by 16. Result is how much of original recipe to make.
2) Do the math. Multiply the length x width x height" (usually 2") for each. That gives you cubic inches. Divide the cu in of the 12 x 17 over that of the 9 x 13 and you get your recipe factor to make.
3) Look at this Alice Medrich article about making cakes fit the pans you have. Your pan size might be in there.
https://food52.com/blog/13239-how-to-make-your-baking-recipe-fit-your-pan-size