A new study found many fast-food consumers are badly mistaken when it comes to the amount of calories they think they’re putting into their body, according to USA Today.

According to Harvard Medical School and Harvard Pilgrim Health Care Institute’s Jason Block, the lead researcher of the study, Teens underestimated the amount of calories put in fast food by 34 percent, the parents of school-aged children underestimated by 24 percent, and adults by 20 percent.

The survey consisted of approximately 3,400 teens, adults, and parents of school-aged children. All of the participants ate at a combined total of 89 fast-food restaurants including the popular chains—Burger King, Subway, Dunkin’ Donuts, Wendy’s, KFC, and McDonald’s.

First, the researchers requested that the participants order a meal and try to guess how many calories were present in that meal. Next, the researchers took the receipts and determined how many calories were present within the meals.

Here are some highlights from the study:

v  Twenty-five percent of the participants guessed below the actual amount of calories by at least 500.

v  The meals ordered by teens’ had an average of 756 calories. They guessed below that amount by an average of 259 calories.

v  Adults’ meals were at around 836 calories, they guess below that number at an average of 175 calories.

v  School-age children had an average of 733 calories in their meals. However, their parents guessed below that number at an average of 175 calories.

v  Those who ate at Subway had the greatest underestimation compared to those who ate at the other five restaurants.

"These large underestimations show that diners don't really know what they are eating in terms of calorie content, and they need this information to help guide their choices," says Block."They could get it from the company websites or in some other form in the restaurants, such as wall posters, napkins or cups, but soon they'll be directly faced with it when they see it on the restaurant menu boards before they order their meal. Customers can already do this at McDonald's -- and in some cities.”

The study was partly sponsored by the Robert Wood Johnson Foundation. The study was conducted in 2010-2011 but was published Thursday in BMJ.