Though calorie counting has fallen in and out of favor over the years as a way to diet, most weight-conscious people do it to some extent. That means comparing the calorie counts shown on food labels and on display at chain restaurants. But what if the calorie counts are inaccurate?

The FDA allows food companies wide latitude in the accuracy of the calories listed on package labels—20 percent in either direction. That means if a label says 200 calories per serving, it could be 240 calories or 160 calories or anything in between. What’s more, the FDA doesn’t do any systematic policing of labels to ensure that calorie counts meet even that lax degree of accuracy. The responsibility for label accuracy remains with the food companies, from national manufacturers to regional or local vendors. It basically works on the honor system.

Where the numbers come from

How do food companies figure out calorie counts? In theory, they should test the food in a lab—putting a sample in an instrument called a bomb calorimeter, a small chamber in which a food is burned to heat water; the hotter the water gets, the higher the calorie count (a calorie is a unit of energy). But more often, companies simply add up the calories of the various ingredients in the foods using a standard nutrient database. That should provide the same counts as the lab analysis, provided that data are accurate.

So how accurate are calorie counts?

Labels usually undercount calories by a little, but sometimes by a lot. Here’s a sampling of findings:

In a study in the journal Obesity in 2013, NIH researchers determined the “true” calorie content of two dozen snack foods (national brands), including chips, candy bars, and cookies, using a calorimeter. They found that the actual calorie content was 4 percent higher, on average, than labeled (mostly because the foods contained more carbohydrates than listed). That’s pretty accurate, as such things go.

In an informal report in the New York Times in 2013, a filmmaker had food scientists analyze five food items that he typically ate: a Chipotle burrito, Subway sandwich, Starbucks Frappuccino, plus a locally distributed muffin and tofu sandwich. The Subway and Starbucks items were fairly accurately labeled, but the other three foods contained far more calories than labeled (the burrito had 1295 calories, not 1175 as labeled; the muffin, 735 not 640 calories; the tofu sandwich, 548 not 228 calories).

Are “healthier” food options less prone to such discrepancies? Apparently not. In a study in the Journal of the American Dietetic Association in 2010, researchers purchased 39 reduced-calorie frozen meals (such as Weight Watchers and Healthy Choice) and foods from chain restaurants (such as Taco Bell, McDonald’s, and Applebee’s). Again, testing in a calorimeter revealed that calorie counts were usually understated. The frozen foods averaged 8 percent more calories than labeled. Meanwhile, restaurant foods averaged 18 percent more calories, with a few actually containing twice as many calories as stated on menus or menu boards.

In a study in the Journal of the American Medical Association in 2011, researchers looked at foods from 42 fast-food and sit-down restaurants in three states. They compared the calorie counts provided in the restaurants with those measured in a lab. Of the 269 foods, one in five contained at least 100 calories more per serving than stated, and one in 10 averaged about 275 extra calories. Discrep­ancies were greater at sit-down restaurants than at “quick-serve” chains, probably due to poorer control of serving sizes in the kitchens. Ironically, relatively low-calorie foods—the ones most likely to attract weight-conscious consumers, like salads and soups—underreported calo­ries more often than those with higher listed numbers.

Where does this leave us?

It’s safe to assume that many food labels and restaurant menus understate calories, often because serving sizes are larger than indicated. In general, research has found that national brand products, which are more carefully standardized, tend to be more accurately labeled than those from local companies.

For most people, the discrepancies in calorie counts are of little consequence, since they don’t know how many calories a day they should be consuming in the first place (see Your Daily Calorie Count). But even if most calorie counts are slightly off, it’s still useful to compare the numbers listed on various brands. On the other hand, for people who have been given daily calorie limits by a nutritionist and are carefully counting their calories, the differences may add up.

Of course, if you prepare and eat mostly whole foods, as recommended (and consume few packaged or restaurant foods), none of this matters much, since such foods generally don’t carry nutrition labeling. In any case, meticulously counting calories hardly guarantees weight loss. A better option is mindful eating—for instance, being aware how much you are serving yourself, how much you are eating, and how full you feel—while focusing on wholesome foods.

Also see 7 Must-Know Calorie Facts.