I plan to make a stonefruit and blueberry crisp for guests tonight. The recipe I will share in another post makes enough to fill a 9" pie plate. I'd like to have leftovers so I will use a 9" by 13" pyrex baking dish. This requires some thought. What multiple of my single recipe to use? There's always finding the baking dish equivalency page in the Joy of Cooking ( this book is the Eighth Wonder of the World).
I do some contorted math in my head eg the surface area of a 9" pie plate can be calculated by the formula pi X radius squared. The radius is 4.5 but multiplying 4.5 times 4.5 in your head is too much. I just multiply 4.5 times 4 which is obviously 18 then add another .5 of 4.5 = 2.25. pi is 3.14159, but, hey, this is just a crisp, so 3.1 works. The area of the plate is about 63 square inches.
The 9" by 13" baking dish is 117 square inches. Doubling the recipe will work fine. A little extra topping is always good.
Then there's always the calculator app on your smart phone.