How accurate are fitness tracker devices?

The question: I'm constantly looking at my fitness tracker. But how much can I really rely on what it's telling me?

The expert: Ray Browning, Ph.D., director of the Physical Activity Energetics/Mechanics Lab at Colorado State University

The answer: It depends what, exactly, you're tracking and which brand you're wearing.

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Why? These fitness devices track everything—from calories burned to steps taken—with their built-in accelerometers. And as the name suggests, they only detect acceleration (changes in motion), not exertion. They don't have any idea if your arm is wielding a candy bar or a 50-pound dumbbell.

That's why, as you may or may not have noticed, your tracker gives you little to no credit for some of your workouts. A lot of strength training exercises—not to mention biking—all read like you're just chilling out on the couch, Browning says. Remember: If your tracker is not bouncing around, it's not counting your exercise.

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In one recent study published in Medicine & Science in Sports & Exercise, trackers' calorie expenditure (one subset of the genre) was found to be between 10 and 15 percent off, on average, which isn't so bad. Researchers asked 30 men and 30 women to complete a 69-minute workout that included 13 different activities—from writing at a computer and playing Wii tennis to running and shooting hoops. They then compared the readings from eight fitness trackers against those from portable (and far more accurate) metabolic analyzers.

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The BodyMedia FIT, Fitbit Zip, and Fitbit One, were the most accurate (with 9.3, 10.1, and 10.4 error ratings, respectively), while the Jawbone Up, Actigraph, Directlife, Nike Fuel Band, and Basis Band, brought up the rear (with 12.2, 12.6, 12.8, 13.0, 23.5 percent error ratings, respectively).

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But if you put too much faith in even the most accurate trackers—basing your calorie intake on how many calories your device says you're burning—you could end up gaining, not losing weight, Browning says. For instance, if your fitness tracker says you've burned 3,000 calories today, you may have actually only burned 2,500.

The smartphone apps for some of these fitness trackers will let you manually enter exercises to get a more accurate calorie-burn total, but even if your tracker knows your height, weight, age, and gender, the calorie-estimate could be off, of course. (the accuracy of cardio machine calorie-counters is another matter.)

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There is a silver lining, though: "These fitness trackers may not be accurate in counting calories, but their results are repeatable," says Browning. If you do the exact same thing two days in a row, you can expect the same tallies from your tracker. That means you can easily use them to track your progress. Have you burned more calories today than yesterday? Taken more steps? You can trust that info.

And progress, in the end, is what will make you fitter and faster.

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