Recipe scaler: by servings or pan size
Paste the ingredient list, then tell it what changed: the number of servings or the pan. Every amount is rewritten in measures you can actually use.
How it works
By servings. The scale factor is new servings ÷ original servings. Eight servings to twelve is a factor of 1.5.
By pan. The factor is the area of the new pan ÷ the area of the original pan. A rectangle is length × width. A round pan is π × radius². An 8-inch square (64 sq in) and a 9-inch round (about 63.6 sq in) are almost the same size, which is why King Arthur Baking treats them as swaps. A 9 × 13 pan is 117 sq in, so going from an 8-inch square multiplies the recipe by about 1.83. Use pans of the same depth.
Rewriting the amounts. Each line starts with a number, maybe a unit, then the ingredient. The scaler multiplies the number, and any gram or ounce weight in parentheses. Then it moves to a friendlier unit: 3 teaspoons become 1 tablespoon, and 4 tablespoons become 1/4 cup. Odd cup amounts are split into cups plus spoons. Lines without a number, like "pinch of salt", are left alone.
Baking time does not scale with the recipe. A deeper or larger pan may need more time, and the center is the last part to set. Start checking at the original time.