Recipe Scaler

Background.

A recipe written for four, a table set for ten. The arithmetic is a single ratio, 10 ÷ 4 = 2.5, but applying it at the counter is fiddly: three-quarters of a cup times 2.5 is 1.875 cups, an amount no measuring cup is marked with, and 1½ teaspoons becomes 3.75 teaspoons, which is easier to measure as a tablespoon plus ¾ teaspoon. Enter one ingredient at a time and you get the scaled amount in the unit you started with, rounded the way you choose, and for cups, spoons, fluid ounces and metric volumes the same amount in millilitres.

Proportional scaling is the right starting point because a recipe is a set of ratios. Flour to liquid, fat to flour, sugar to everything else: multiply every ingredient by the same factor and those ratios survive, which is why a doubled stew tastes like the original. What does not scale with the ingredient list is the cooking. A cake batter doubled into the same tin is twice as deep and bakes differently; a pot holding two and a half times the soup takes longer to come to the boil; a big batch of dough is harder to mix by hand. Salt, strong spices and leavening deserve a taste or a test after a large change, and tested preserving recipes should not be rescaled casually at all.

The millilitre figures follow the US nutrition-labelling definitions in 21 CFR 101.9, where a teaspoon is 5 mL, a tablespoon 15 mL and a cup 240 mL, plus 29.5735 mL for a US fluid ounce. Grams, ounces, pounds and counted items get no millilitre line, because turning weight into volume needs the density of the particular ingredient: USDA's FoodData Central lists a cup of all-purpose flour at 125 g, a cup of granulated sugar at 200 g and a cup of whole milk at 244 g.

What is recipe scaler?

The scaling factor is the number every ingredient is multiplied by: desired servings divided by original servings. It is a pure ratio with no unit, so it works the same on cups, grams, pounds or eggs, and it runs in either direction: 2.5 to go from four servings to ten, 0.5 to halve a recipe for four into one for two, 1.6667 to stretch a six-serving recipe to ten. A factor of 1 leaves everything as it was.

The servings are only a way of expressing that ratio. Any two numbers in the right proportion give the same factor, which makes the fields useful for scaling by something other than people: a pan's area, a number of loaves, a percentage. Bakers working in baker's percentages rely on the same idea, writing every ingredient as a percentage of the flour weight so that choosing a new amount of flour fixes everything else. Scaling keeps the proportions and changes the total; unit conversion keeps the amount and changes the unit. This calculator does the first, and for volumes it adds one piece of the second.

How to use this calculator.

1. 01Enter Original servings as printed on the recipe and Desired servings as the number you need; both must be at least 1. To halve a recipe that serves four, enter 4 and 2.
2. 02Type the Ingredient quantity as a decimal: 1½ becomes 1.5 and ¾ becomes 0.75. Enter a third as 0.3333 rather than 0.33, which is a full percent short.
3. 03Pick the Ingredient unit the recipe uses. Ounce means a weight ounce, so choose Fluid ounce for liquids measured by volume; Count is for eggs, garlic cloves, lemons and anything else you count rather than measure.
4. 04Leave Show metric volume equivalents on Yes if you would rather pour liquids into a metric jug. The millilitre line appears only for teaspoons, tablespoons, cups, fluid ounces, millilitres and litres.
5. 05Choose a Rounding style that matches how you will measure: Exact decimal for a scale or metric jug, Fraction for cup and spoon sets marked in eighths, Practical kitchen rounding for quarters and whole grams or millilitres. Then change the quantity and unit for the next ingredient; the servings stay as they are.

The formula.

Two lines carry the whole method: S = D ÷ O, then Qty′ = Qty × S. O is the number of servings the recipe makes, D the number you want, Qty the ingredient amount and Qty′ the scaled amount, in the same unit as Qty. Nothing else touches the scaled quantity unless you ask for rounding.

Rounding changes only the displayed quantity. Fraction mode moves cups, tablespoons, teaspoons, fluid ounces, ounces, pounds and counts to the nearest 1/8 and leaves grams, millilitres and litres exact. Practical kitchen rounding moves the same household units to the nearest 1/4, rounds grams and millilitres to whole numbers, and leaves litres exact. An amount exactly halfway between two steps goes up, so 1.875 cups, midway between 1¾ and 2, shows as 2 under kitchen rounding. An amount small enough to round to zero is shown unrounded instead, so a pinch never vanishes from the list. Results always appear as decimals; a fraction-mode answer of 1.875 is 1⅞ on a measuring cup.

The millilitre line comes from the exact scaled amount, never the rounded one, using these factors: teaspoon 5 mL, tablespoon 15 mL and cup 240 mL, the 21 CFR 101.9 labelling definitions; US fluid ounce 29.5735 mL; millilitre 1; litre 1,000. The labelling values are rounder than the US customary measures, in which a cup is 236.588 mL and a tablespoon 14.787 mL, so cups and spoons converted here read about 1.4% higher than a customary conversion would. The fluid ounce uses the customary figure, which means 8 fl oz converts to 236.588 mL while 1 cup converts to 240 mL. Neither gap is big enough to change how a dish turns out, but it explains why two converters can disagree by a few millilitres. Weight units and counts never get a millilitre figure, and nothing here converts between weight and volume.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Electronic Code of Federal Regulations (2026). '21 CFR 101.9: Nutrition labeling of food.' U.S. Government Publishing Office and National Archives.
2. [2]U.S. Food and Drug Administration (2024). 'The Nutrition Facts Label.' FDA.
3. [3]U.S. Department of Agriculture (2026). 'FoodData Central.' USDA Agricultural Research Service.
4. [4]National Institute of Standards and Technology (2026). 'NIST Handbook 133, Appendix E: General Tables of Units of Measurement.' U.S. Department of Commerce.
5. [5]National Institute of Standards and Technology (2008). 'Guide for the Use of the International System of Units (SI), NIST Special Publication 811.' U.S. Department of Commerce.