On Cereal Serving Sizes. Grape Nuts as an Example

This is a guest blog post by Richard Perlmutter, MS

A serving of Post Grape-Nuts ready-to-eat breakfast cereal weighs 58g.
A serving of Post Grape-Nuts Flakes ready-to-eat breakfast cereal weighs only 29g.



The federal Food and Drug Administration (FDA) has rules that determine the serving size amounts that appear on the Nutrition Facts panel of packaged foods and beverages.

For most products these rules insure that different products in the same category of food have the same or a similar serving size. With similar sizes it is easy to make nutrition comparisons among similar products. But FDA rules for ready-to-eat breakfast cereal do not follow this reasoning.

Some breakfast cereals feel heavy and others feel light. But ‘heavy’ and ‘light’ are actually misnomers. It is more accurate to describe them as denser or less dense, where density refers to the weight of a specific volume of cereal. Density is the main characteristic used to determine the serving size.


If a cereal has a low density–less than 20g per cup–a serving is about 15g.

If the cereal has a medium density–20g or more but less than 43g per cup–a serving is about 30g.

If the cereal has a high density–43g or more per cup–a serving is about 55g.

A cup of Grape-Nuts weighs about 116g. A cup of Grape- Nut Flakes weighs about 39g.

Serving sizes are expressed in common household measures. For ready-to-eat breakfast cereals these are fractions of a cup. But they must be easy to measure fractions like 1/4, 1/3, and 1/2. Not difficult ones like 7/8, 7/16, or 9/32.

The actual weight per serving is the weight of the fraction of a cup that is closest to the recommended serving size based on the density of the cereal. Breakfast cereals are manufactured in a broad range of densities, which makes defining a serving size uniquely challenging. For nutritional comparisons it is best that they all have the same weight. But consumers fill a bowl. As served, the amount is a volume.

FDA rules, if you will, split the difference.

Richard Perlmutter is the owner of Abington Nutrition Services LLC which prepares nutrition labeling for products manufactured by food and beverage companies. He also takes an interest in seeing that government nutrition policy is in line with nutritional science.

(References to this article are available on request from the author.)

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