Serving size: why the cup and the grams on a label disagree

Turn over a bag of penne and the serving line says something like "3/4 cup (56g)". Turn over a bag of lentils and it says "1/4 cup (35g)". A bag of shredded mozzarella says "1/4 cup (28g)". Each line reads like a measurement: this much in a cup, weighing this much on a scale.

It is not a measurement. A serving line is built the other way round. The grams come first, from a table of standard amounts that US labelling rules set for each kind of food, and the cup is then rounded to the nearest fraction the rules allow. That is why the cup on the label and the cup in your kitchen can disagree by a third or more.

How a serving size is made

The FDA sets a reference amount for each category of food, meant to reflect what people eat at one sitting. For dry pasta it is 55 grams. For dry beans and lentils it is 35 grams. For cheese it is 30 grams. The manufacturer then has to express that amount in a household measure, and the rules fix the arithmetic: for labels, 1 ounce means 28 grams, and cups have to be written in quarter or third steps (1/4, 1/3, 1/2, 2/3, 3/4, 1).

So pasta makers label the 55 gram reference amount as 2 ounces, which the rules turn into 56 grams, and add the cup fraction that comes closest. Bulk cheese is labelled as 1 ounce, 28 grams. Dry beans keep the 35 grams. Nobody is required to fill a cup and weigh it.

Pasta: one weight, many cups

Every bag of dry pasta in an American supermarket gives the same 56 gram serving, and the cup beside it changes with the shape and with the brand. Prince prints 1/3 cup for orzo, 1/2 cup for elbows, 3/4 cup for penne and a full cup for bow ties, all at 56 grams.

For orzo the label cup is close to the truth: 1/3 cup at 56 grams works out to about 168 grams a cup, and America's Test Kitchen and USDA weigh a cup of orzo at 170 to 191 grams. For penne it is not. Prince says 3/4 cup, Ronzoni, Mueller's and Kroger say 2/3 cup and De Cecco says 1/2 cup, for the same 56 grams of the same shape. USDA weighs a cup of dry penne at 95 grams, so 56 grams is about 0.6 of a cup. None of the four labels measured it: they round the same weight up to 3/4 or 2/3 of a cup, or down to 1/2.

Shapes that trap air spread the numbers further. Bow ties weigh 60 to 81 grams a cup depending on how they settle, and egg noodles can weigh less than 40 grams a cup when they are loosely filled. The side-by-side figures for every shape are on the pasta comparison.

Dry beans: the quarter cup is heavier than it says

"1/4 cup (35g)" is printed on most store-brand bags of dry beans and lentils. A level quarter cup of dry lentils weighs about 48 grams; USDA puts a full cup at 192 grams, and the Harris Teeter label prints 48 grams for the same quarter cup. Navy beans come to about 52 grams a quarter cup.

So the 35 grams on the bag is about three tablespoons, not a quarter cup. If you cook a quarter cup per person and read the nutrition panel as one serving, you are eating about a third more than the panel describes. Other brands make their own choices: Camellia prints "App. 1/4 cup (51g)", which is simply a ninth of a pound. Our beans and lentils comparison gives the measured cup for twelve of them.

Shredded cheese: the quarter cup is lighter than it says

Cheese runs the other way. Almost every bag of shredded cheese says a quarter cup weighs 28 grams, because 28 grams is the one-ounce label statement the rules set for cheese sold in bulk. USDA weighs a cup of shredded part-skim mozzarella at 86 grams, about 22 grams a quarter cup (shredded mozzarella). The 28 gram serving is nearer a third of a cup. Sprinkle a level quarter cup and you have used about a fifth less cheese than the panel counts.

The cup on the label is not your cup either

There is one more gap underneath the others. For labels, a cup means 240 millilitres. The measuring cup in an American kitchen holds the US customary cup, 236.6 millilitres. The difference is one and a half percent, small next to the errors above, but it is why a label's cup and a measured cup never line up exactly. We wrote about it in the label says grams, the recipe says cups.

Which number to trust

• Trust the grams. The gram figure is the serving the nutrition panel describes. The cup next to it is an approximation the rules allow.
• Weigh when it matters. If you track what you eat, weigh the serving the label names. A kitchen scale settles it in a second.
• Measure with a measured figure. If you cook with cups, use a figure from someone who filled a cup and weighed it. Every ingredient page on ExactCups names its sources and shows where label servings were left out because they are servings, not measurements.
• Compare products per 100 grams. Serving sizes differ between brands, so the per-100-gram column is the fair comparison, and it is what grocery scanner apps build their scores on (six scanner apps compared).

That last point is where SwapGenie, the app we build alongside ExactCups, comes in. It reads the barcode, checks the product against your own allergens and diet, and suggests a better version of the same thing to buy instead. SwapGenie tells you what is in the packet; ExactCups tells you how much of it is in your cup.

Frequently asked questions

How many cups is 56 grams of dry pasta?

It depends on the shape. 56 grams is about 1/3 cup of orzo, about 1/2 cup of elbow macaroni, a little over 1/2 cup of penne and about 2/3 to 1 cup of bow ties. Pasta labels all use 56 grams, two ounces, and round the cup to the nearest fraction, so two brands of the same shape can print different cups.

Is a quarter cup of dry beans 35 grams?

No. 35 grams is the FDA reference amount for dry beans, not the weight of a quarter cup. A level quarter cup of dry lentils or beans weighs about 42 to 52 grams, so 35 grams is closer to three tablespoons.

Why does shredded cheese say a quarter cup is 28 grams?

Labels for cheese sold in bulk state the serving as one ounce, which labelling rules define as 28 grams. USDA weighs a cup of shredded part-skim mozzarella at 86 grams, so a level quarter cup weighs about 22 grams and 28 grams is nearer a third of a cup.

Why is a cup 240 mL on a nutrition label?

US labelling rules define a cup as 240 millilitres, a tablespoon as 15 and a teaspoon as 5, to keep the arithmetic round. A US measuring cup holds 236.6 millilitres, about 1.5 percent less.

Should I trust the cups or the grams on a food label?

The grams. The nutrition panel describes the gram weight; the cup beside it is an approximation rounded to a quarter or a third. For cooking by volume, use a cup weight that someone has measured.