Lead researcher Dr. Jason Block of the Harvard Medical School/Harvard Pilgrim Health Care Institute colleagues surveyed nearly 3,400 adults, teens and school-age children in 2010 and 2011 at 89 fast-food restaurants in four New England cities.
"Teens underestimate the number of calories in their meals by as much as 34 percent, parents of school-age children by as much as 23 percent, and adults by as much as 20 percent," Block said in a statement.
The study, published in the British Medical Journal, found adults ordered meals containing 836 calories on average but typically underestimated the calorie content of those meals by 175 calories.
However, teens had the largest gap. They underestimated the number of calories in their average order, which contained 756 calories, by 259 calories, the study said.
Parents of school-age children underestimated their orders, which contained an average of 733 calories, by 175 calories overall.
One-quarter of all participants in the study underestimated the calorie content of their meals by at least 500 calories, Block said.