How recipe scaling works
Scaling by servings is simple division: desired servings divided by original servings is your multiplier, and every ingredient gets multiplied by it. A 12-serving recipe made for 18 people is 18 / 12 = 1.5, so 2 1/4 cups of flour becomes 3 3/8 cups and 2 eggs become 3. Halving a recipe is a multiplier of 0.5, doubling is 2. The multiplier option lets you type any factor directly.
Pan conversions are the part people get wrong, because batter volume follows area, not diameter. A round pan's area is pi times the radius squared, so the ratio between two round pans is the ratio of their diameters squared. Going from a 9-inch to an 8-inch round pan is 8 squared over 9 squared = 64 / 81 = 0.79, not 8 / 9 = 0.89. Rectangular pans use width x length: a 9x13 pan is 117 sq in and an 8x8 pan is 64 sq in, so the multiplier is 64 / 117 = 0.55, or roughly half. Going the other way, 8x8 to 9x13, is 1.83 times. Any consistent unit works for pan sizes; the ratio is the same in inches or centimeters.
How the parser reads your recipe
Paste one ingredient per line, with the amount first, the way most American recipes are written. The tool recognizes whole numbers, decimals, plain fractions like 3/4, mixed numbers like 1 1/2, and typographic fractions such as 1/2 or 3/4 typed as single characters. Everything after the number is kept as is, so "1 cup butter, softened" stays readable. Lines that do not start with a number, like "pinch of nutmeg" or "salt to taste," are passed through untouched and counted in a note under the result.
Results under 10 are shown as kitchen-friendly fractions, snapped to the nearest half, third, quarter or eighth when that is within 4% of the true value; otherwise you get two decimals. Amounts of 10 or more are rounded to one decimal. That means 2 1/4 cups x 1.5 prints as 3 3/8 cups, 1 tsp x 1.5 prints as 1 1/2 tsp, and 3/4 cup x 0.55 (0.41 cup) snaps to 3/8 cup, which is 6 tablespoons. Use the Copy button to paste the scaled list into your notes.
What does not scale in a straight line
Leavening, salt and spices scale reasonably well up to about 2x, but when you triple or quadruple a recipe start with a bit less baking powder, baking soda and salt and adjust to taste. Eggs are the classic problem: 2 eggs x 1.5 = 3 works, but 2 x 0.79 = 1.58 does not. Beat an egg, weigh it (about 50 g) and use the fraction you need, or round to the nearest whole egg for anything forgiving like muffins.
Baking time and temperature do not multiply. A bigger or deeper pan needs the same oven temperature and 10-25% more time; a smaller or shallower one needs less. When you scale a cake for a larger pan, keep the batter depth similar to the original and check for doneness early. Cooking times for stews and braises hardly change at all when you double them, since the heat still has to reach the center of each piece of food.