The food at many popular chain restaurants and in the freezer section of the supermarket may have a lot more calories than advertised.

A study of 10 chain restaurants found that the number of calories in 29 menu items was an average of 18 percent higher than listed.

And frozen supermarket meals from Lean Cuisine, Weight Watchers, Healthy Choice and South Beach Living had 8 percent more calories than the labels said, according to the study in this month's Journal of the American Dietetic Association.

The researchers aren't accusing restaurants and food companies of trying to deceive customers. They said most of the discrepancies can be explained by variations in ingredients, portion sizes and testing methods. For example, the teenager behind the counter might put too much mayonnaise on one sandwich.

Still, "if every time you eat out, you get a couple of hundred calories or more than you think, that can add up really easily," said lead researcher Susan Roberts, of Tufts University.

Most of the packaged food tested fell within the 20 percent margin of error allowed by the government.

Some restaurant items, like a Domino's large thin-crust cheese pizza, came in low. It had one-third fewer than the reported 180 calories per serving.

Wendy's Ultimate Chicken Grill was found to have 9 percent more calories than its reported 320. P.F. Chang's large Sichuan-style asparagus had more than double the 200 calories it claimed. A Ruby Tuesday's baked potato with butter and sour cream came in on target, but 3 percent more calories were measured in a McDonald's McChicken sandwich, said to have 360 calories.