Originally Posted by KoryAK
There's a far easier way: math! If the recipe you have fills an 8x8 square, then that's 64 square inches. If the pan you need to fill is 26x18, then you need 468 square inches. 468 divided by 64 is 7.3. You need to multiply your recipe by 7.3 (or you can round up to where the numbers make a bit more sense and you aren't using .35 eggs or something)
This would be true, if you're comparing that your recipe is enough to fill an 8x8x1 square pan about 3/4 of the way full.
The 8x8 square pan is only 64 surface
square inches... if it's 2" deep and filled to bake a 2" tall layer, it's 128 square inches.
The 18x26 pan is 468 square inches divided by 128 is 3.66, so you'll need to multiply your recipe by almost 4.
It's best to multiply the actual cups that your batch makes, because you probably aren't completely filling the pans to the top (unless you're using extenders).