How the scaling works
new amount = original amount x (servings you want / servings in the recipe)
A recipe for 24 cookies scaled to 36 has a factor of 36 / 24 = 1.5. Every ingredient is multiplied by 1.5, then rounded for the kitchen:
- Cups, tablespoons, teaspoons and fluid ounces become US cups and spoons, to the nearest 1/8 teaspoon. Cup fractions come from a standard set (1/4, 1/3, 1/2, 2/3, 3/4), so 1 1/2 x 3/4 cup reads 1 cup + 2 tbsp.
- Grams round to 0.5 g under 10 g, to 1 g up to 100 g, to 5 g up to 1 kg and to 10 g above that.
- Ounces and pounds round to the nearest quarter ounce and show pounds and ounces together.
- Millilitres round to 0.5 mL under 10 mL, 5 mL up to 250 mL, 10 mL up to a litre and 50 mL above.
- Eggs and other whole items round to the nearest half. For half an egg, beat one and use half of it by weight.
Every spoon conversion uses the exact US definitions in NIST Handbook 44: 3 teaspoons to a tablespoon and 16 tablespoons to a cup.
Halving and doubling the awkward amounts
| Recipe says | Half | One and a half | Double |
|---|---|---|---|
| 1 cup | 1/2 cup | 1 1/2 cups | 2 cups |
| 3/4 cup | 1/3 cup + 2 tsp | 1 cup + 2 tbsp | 1 1/2 cups |
| 2/3 cup | 1/3 cup | 1 cup | 1 1/3 cups |
| 1/2 cup | 1/4 cup | 3/4 cup | 1 cup |
| 1/3 cup | 2 tbsp + 2 tsp | 1/2 cup | 2/3 cup |
| 1/4 cup | 2 tbsp | 1/3 cup + 2 tsp | 1/2 cup |
| 1 tbsp | 1 1/2 tsp | 1 tbsp + 1 1/2 tsp | 2 tbsp |
What doesn't scale in a straight line
Ingredient amounts scale by the factor; some other parts of a recipe do not.
- Pan size. Doubling a cake recipe needs twice the pan area, not a pan twice as deep. A deeper layer bakes differently.
- Baking time. It follows the depth of the food, not the amount of batter. More cookies on more trays take the same time each; one larger loaf takes longer.
- Salt, spices and leaveners. These scale by the same factor on paper. For large changes, some cooks prefer to start a little under and adjust to taste, which is easiest with salt and spices.
- Liquids that evaporate. A sauce reduced in a wider pan loses more water.
Scale by weight when you can
Spoon and cup rounding introduces a small error on every line; weights do not. If you are scaling a baking recipe by an uneven factor, convert it to grams first with the grams to cups converter, scale the grams, and weigh. NIST's kitchen tips make the same point: weighing with a kitchen scale is more accurate than measuring by volume.