Next time you go out for that big burger, bucket of fries and super-sized soda, bring along your calculator.

Most people underestimate the number of calories in fast-food meals — a big problem as portion sizes have ballooned, a new study shows. People make more accurate guesses when the meals are smaller, according to the results of the study being published Tuesday in the Annals of Internal Medicine.

"This is not an issue of knowledge, of motivational biases that people want to lie (about what they eat). It's just ingrained perceptual bias that we can't control," said Pierre Chandon, a co-author of the study and assistant professor of marketing at the international business school INSEAD, based in France.

The study was broken into two parts.

First, researchers asked 105 people eating at fast-food restaurants in three Midwestern U.S. cities to estimate the number of calories in the meals they had just eaten.

In the second part of the study, 40 undergraduate students were asked to estimate the calorie content of 15 various sizes of fast-food meals. The meals, ranging from 445 to 1780 calories, consisted of varying amounts of chicken nuggets, fries and soda.

The results were similar no matter how much the participants weighed or whether they were male or female, the researchers found. However, overweight people in the first part of the study tended to buy larger meals.

"What this study shows is it's the amount of food on the plate that's fooling people," said Dr. Madelyn Fernstrom, director of the University of Pittsburgh Medical Center's Weight Management Center.

Fernstrom, who was not involved in the study, said that as portions have gotten larger, it's become harder for people to estimate what a standard portion should be. The amount people should eat seems puny compared to the mounds of food we have become used to seeing on our plates, she said.

"This is showing human foibles. It's hard to estimate food. And it's really hard to estimate huge portions," Fernstrom said.

Doctors say there are many strategies people can use so they don't underestimate their calorie intake.

Chandon, speaking by phone from Paris, suggested people divide their food into different components to try to determine the calories, instead of looking at a large meal all together.

Fernstrom suggests people eat smaller portions, use a smaller plate so the meal looks larger, and downsize — not super-size — meals when they eat out.

"The idea here is that you can always get more food" if you are hungry, Fernstrom said. "If you cut your portions down, at least you have a fighting chance."