Portion sizes refer to the amount of food you eat. Serving sizes are what appear on Nutrition Fact Labels on food packages. And there can be a big difference—which matters when you're trying to control your weight.

What counts as a serving for a particular food is determined by the Food and Drug Administration, but serving-size standards are based on the amounts Americans typically ate roughly 30 years ago. For instance, a 4 ½ ounce muffin would be about 2 servings because the FDA’s serving size for muffins is about 2 ounces. If you glanced at the nutritional information on the muffin’s package without paying much attention to the serving size, you might think the entire muffin has fewer calories and less fat and sugar than it does.  

Americans’ portion sizes are consistently greater than current serving sizes, as found in a recent Consumer Reports experiment. We asked 124 supermarket shoppers to pour a cereal serving into their choice of three different sized bowls. The portion sizes were much larger than the serving sizes. The shoppers put at least twice the recommended serving into the largest bowl; 92 percent overpoured regardless of the bowl size. 

The FDA wants to update serving sizes on food labels so they better reflect the amounts Americans typically consume today. That may help people better control how much they eat. But people often interpret the serving size as the amount they should be eating, and some experts worry that adjusting serving sizes up will just encourage people to eat more.

What may be most helpful is simply making label reading a habit. “You wouldn’t go shopping without looking at a price tag,” says Jean Kristeller, Ph.D., author of “The Joy of Half a Cookie” (TarcherPerigree, 2015) and professor emeritus of psychology at Indiana State University. Checking nutrition labels should be automatic, too. Then get real about how those serving sizes compare to the portion sizes you typically eat