Calorie counts on restaurant menus are suppose to help fight America's obesity problem. But a new study from Tufts University found many of those counts are not accurate.

Researchers studied 42 restaurants and found that 20 percent of the foods tested had 100 calories or more over what was stated on the menu.

The study also found that fast food restaurants were the most accurate because of the uniform recipes and portions, but there were wide variations found in sit-down restaurants.

Experts say that while an extra 100 calories in each meal may not sound like a whole lot, it adds up. Lorien Urban, a postdoctoral associate in the energy metabolism lab at Tufts University says consuming an extra 100 calories per day can lead to an extra 10 pounds in one year.

Eleven meals had more calories than on the menu and 10 had fewer calories. Some were over by only 40 calories; another was over by as much as 420 calories.