Fast-food customers who order large or super-sized meals consume about 500 calories more than they think.
Given that Americans eat an average about three meals a week, 500 uncounted calories at each one can add up very quickly.
Over 100 adults were asked to estimate the calorie content of the meals they had just consumed at a variety of restaurants, including McDonald's and Subway.
Participants were also asked their height and weight. The customers' estimations were then compared to the actual calorie content.
The study found that people who ate larger meals estimated they had eaten an average of 675 calories, but had actually consumed 1,188, a 513-calorie difference.
In a second study, 40 undergraduates estimated the calories in 15 meals with different-size portions of chicken nuggets, french fries and Coca-cola. The students guessed that the larger meals contained roughly 1,000 calories, when in actuality they contained more than 1,300.
The studies also showed that overweight people are more likely to order bigger meals than those at a normal weight.