When bleary-eyed early-risers pour their morning cereal, they typically skip the measuring cup to stay in line with the serving sizes listed on the side of the box.
People also rarely stop at a handful of chips or one pretzel.
That's why the U.S. Food and Drug Administration is considering tweaking its guidelines on the serving sizes listed on nutrition labels, recognizing that portion-control recommendations are outdated, given Americans' eating habits.
"These servings sizes, though, are based on data, surveys that were done in the 1970s or '80s," ABC News' senior health and medical editor Dr. Richard Besser told "Good Morning America" today.
As a result, Americans often unkowningly consume more calories and sodium than they need, sometimes almost their entire recommended daily allowance in one sitting.
"You can't blame the manufacturers," Besser said. "The FDA tells them exactly what the servings size is, based on these surveys. That's why the FDA needs to take this on."
Serving sizes on food labels even confuse the experts, according to a New York Times report.
A typical portion size for chips is about one ounce, or six chips. For cereal, maybe a half or three-quarters of a cup.
"To consumers, the serving size appears to be inconsistent and unintuitive," Wendy Reinhardt Kapsak, senior director of health and wellness at the International Food Information Council Foundation, told the Times. "They have trouble trusting it."
The question is, Besser said, whether to show people the nutrition content for the serving size the average American actually eats or the portion they should be eating to stay healthy.
"Who eats half a muffin as a serving?" he said. "But that's the serving of a muffin.
"Most people, they're sitting and watching a game, they like to have a bowl of chips."
And when it comes to frozen pizzas, many Americans think nothing of eating the entire personal pizza, even though the serving size indicates nutrition information for one-third of the pie.
"If you put on a meaningful portion size, it would scare a lot of people," Barry Popkin, a nutrition professor at the University of North Carolina, told The New York Times. "They would see, 'I'm going to get 300 calories from that, or 500 calories.'"