How we calculate

Last reviewed 2026-11-03

This page sets out exactly what the tools calculate, so you can check any result by hand. Calculations run in your browser using one shared set of formulas, which is covered by automated tests against independently worked answers.

Scaling a recipe

Scale factor = new yield ÷ original yield. Every ingredient amount is multiplied by that one factor, in its own unit. Scaling never switches units by itself.

• Both yields use the same unit (servings, cookies, loaves...), so they always match.
• Amounts can be whole numbers, decimals (1.5, .5), fractions (3/4), mixed numbers (1 1/2) or the fraction characters ½ ⅓ ¼ ¾ and similar. Ranges such as "2-3", commas, and negative numbers are rejected with a message rather than guessed.
• An amount of 0 stays 0. Rows marked "not scaled" are shown unchanged.
• Numbers inside ingredient names are never changed.
• Oven temperature and baking time are never multiplied; they are outside the calculator.

Limits

• Yields: greater than 0 and up to 10,000.
• Scale factor: from 0.01 to 100. Below 0.25 or above 4, a warning says pans, mixing, leavening and timing are more likely to need adjusting.
• Amounts: up to 100,000 in the unit entered. Fractions: denominators up to 64.
• Up to 50 ingredient rows.

Rounding and fractions

Amounts are stored at full precision. Rounding happens only when they are displayed or printed.

• Fraction display uses halves, thirds, quarters and eighths. If the nearest of these is not exact, it is marked "≈".
• If the nearest fraction is more than 5% away (1/6 cup, for example), the decimal is shown with a breakdown into cups, tablespoons and teaspoons of the same system: 1/6 cup (US) = 2 Tbsp + 2 tsp. Teaspoons are rounded to the nearest 1/8 teaspoon, and the breakdown is marked "≈" if that changed anything.
• Decimal display: grams and millilitres to one decimal place under 10 and whole numbers above; ounces to one decimal place; cups and spoons to two decimal places. A non-zero amount is never shown as 0.

Eggs

Eggs are multiplied like everything else and never rounded for you: 3 eggs halved is 1½ eggs. When the result isn't whole, the scaler suggests beating the next whole number of eggs and using a stated share of the beaten weight (for 1½ eggs: beat 2, use 75%). No universal egg weight is assumed. More in what to do with half an egg.

Units we support

Every cup and spoon is labelled with its system. US and metric measures are never mixed without saying so.

The 240 mL "cup" used on US nutrition labels is a labelling convention and is not offered as a measuring cup. Weight-to-weight and volume-to-volume conversions between these units are exact and don't depend on the ingredient.

Cups to grams

Converting a volume to a weight needs the ingredient's density. Each ingredient uses one published reference, turned into grams per millilitre: density = reference grams ÷ reference volume in mL. Then grams = mL × density, and mL = grams ÷ density.

• Reference weights come from USDA FoodData Central (SR Legacy). Where King Arthur Baking's chart gives a different figure, both are shown.
• Metric-cup and Australian-spoon results use the same density. They are calculated from the US-cup figure, not measured separately.
• Results that use an ingredient weight are always marked "≈". How you fill the cup can change the weight more than the difference between sources.
• Ingredients where the published sources disagreed too much were left out: rolled oats (sources about 33% apart) and baking soda (about 30%).

Ingredient reference values

What we don't calculate

• Baking time or oven temperature for a resized recipe.
• Pan sizes (a pan size tool may come later, with its own method page).
• Ingredient substitutions, allergy checks or nutrition.

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