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Yet popular as fitness trackers have become since someone invented the first calorie counter in 1977, they may not be the right tool to help you reach your workout goals.

Burn Notice—Why It Matters

In a lab setting, researchers rely on highly specialized monitors like an electrocardiogram (ECG or EKG), which measures heart rate, and the metabolic cart, which measures oxygen exchange, to calculate calories burned, says Mark Gorelick, assistant professor of kinesiology at San Francisco State University.

But calorie counters in the real world need to rely on more simple measurements (unless dragging one of these into the gym sounds like fun). So the treadmill at your gym asks for your weight and age (and perhaps a few other stats) to predict how many calories you burn while exercising rather than actively measuring complex data, explains Laura Streeper, a fitness specialist at the Human Performance Center at the University of California, San Francisco.

Whatever data the monitor doesn’t ask for, a value is automatically assumed—based on piles of research, of course, but still not tailored to you. And since calorie burn is also affected by weight, gender, age, body composition, and exercise intensity, the number is never going to be 100 percent accurate.

As for machines that track heart rate: They may not be using your heartbeat to calculate calorie burn, relying on pace instead. Factor in how a machine can’t necessarily tell how much we’re actually participating (a.k.a. not hanging on to the handle bars for dear life), and these machines may overestimate calorie burns for fitter, more skilled exercisers, Streeper says.

On the other hand, most fitness bands are at least fairly accurate, with error ratings ranging from 9.3 percent (comparable to lab monitors, researchers say) to 23.5 percent in a study of eight activity monitorsValidity of consumer-based physical activity monitors. Lee, J.M., Kim, Y., Welk, G.J. Medicine & Science in Sports & Exercise, 2014 Sep;46(9):1840-8.. According to this research, the BodyMedia FIT armband may be the most accurate choice.

The Takeaway

Wearable monitors that directly measure heart rate and other data are your best bet, Gorelick says. Then comes calculators on exercise equipment, and a distant third place goes to calculators on websites and apps, which use no direct feedback at all. In general, “the more parameters you can have, the more accurate your equation could be,” Gorelick says.

But it may not matter. For those aiming to lose weight, calorie calculators can be a helpful tool, yet they’re far from the only tool. Heart rate and exercise intensity (as measured by the “talk test”) can be equally effective for gauging a good workout. Instead of relying on pure numbers, focus on lifestyle changes that will naturally burn more calories (like taking the stairs or riding a bike to work). And for those bigger goals—say running a 10K or losing 10 pounds—consider talking to a trainer or physician to get cracking on a more specific master plan.

Originally published January 2012. Updated June 2015.