Banana bread is a ratio before it is a recipe. Fix the number of bananas and everything else falls out — flour, sugar, butter, the works.
Most recipes don't see it that way. They call for "four medium bananas" and leave you stranded the morning you've got two enormous bananas going soft on the counter. This calculator starts from the bananas you actually have: tell it how many (or weigh the mash, if you're feeling precise) and it hands back the full ingredient list plus the right vessel to bake it in. One banana becomes a few muffins. A standard loaf wants three. A pile of them turns into a sheet cake.