LOS ANGELES — Dieters beware: Offerings at popular restaurants may have more calories than what’s stated on menus or company websites.

A team of scientists purchased items from 42 fast-food and sit-down eateries in Indiana, Arkansas and Massachusetts then measured the calories they contained.

The list of stops on their calorie-busting tour included Burger King, Olive Garden, Outback Steakhouse, McDonald’s, Taco Bell and Chuck E. Cheese.

Only 7 percent of the 269 foods tested were within 10 calories of what they were supposed to be, the scientists found. And almost 20 percent packed at least 100 more calories than what was indicated. Over the course of a year an extra 100 calories daily can add up to 10 to 15 pounds, said study lead author Susan Roberts, a senior scientist at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University.

The biggest discrepancies occurred in sit-down restaurants, where the stated calorie information for a menu item and what the researchers measured was off by an average of 225 calories, according to the study, which was published Tuesday in the Journal of the American Medical Association. At fast-food restaurants, the average discrepancy was 134 calories per menu item.

“Restaurant food is hand-prepared, which can create some variation,” said Joy Dubost, director of nutrition and healthy living at the Washington, D.C-based National Restaurant Association, a trade group.

The most variable foods included an order of three pieces of dark chicken meat at Boston Market – listed as 358 calories but packing more than 500 in the study – and the cranberry pecan chicken salad at the Midwestern restaurant chain Bob Evans, listed as 841 calories but weighing in at over 1,100 calories – more than half of what an average adult should eat in a day.

This is especially concerning for people trying to control their weight, said Kelly Brownell, director of the Yale University’s Rudd Center for Food Policy and Obesity, who was not involved in the study.

The researchers later repurchased and retested 13 of the 17 menu selections with the greatest calorie discrepancies – and found that these items were often repeat offenders.

For example, Chipotle Mexican Grill’s burrito bowl, which is supposed to be 454 calories, was 703 calories on one occasion and 567 on the second.