Why calorie counters on treadmills and elliptical machines aren't as accurate as you may think

• If you're trying to lose weight, don't trust the calorie counters on ellipticals, treadmills, and other exercise machines.
• Research shows that ellipticals can overestimate the number of calories burned in a 30-minute workout session by more than 100 calories. Other machines may have similarly inflated numbers.
• Calculating the exact number of calories burned requires measuring the energy expended by individual cells in the human body, which no gym machine can do.
• Stationary bikes typically have the most accurate readouts, followed by stair climbers, treadmills, and ellipticals.

Following is a transcript of the video.

Narrator: We've got some bad news. You could be burning fewer calories at the gym than you think. In a study published in 2018, researchers found that some ellipticals overestimate calories burned by as much as 130 calories per 30 minutes of exercise. To put that in perspective, if you ate an extra 130 calories of food each day, you'd put on more than a pound each month or 13.5 pounds a year! And as it turns out, ellipticals aren't the only machines with imprecise calorie counters.

Mikael Mattsson: When you talk about the calculations here they are way off. They are so much off that you can't really use the information. Meaning that it can be more than 50 percent off.

Narrator: Now for some, counting calories isn't the point. They're in it for the other benefits exercise offers. But for the nearly 50% of Americans who have, at some point, tried to lose weight, it really does matter. That's because research suggests that people might eat more when they think they're burning more calories.

Mikael Mattsson: The participants in the group that had fitness trackers, Fitbit monitors, they actually didn't lose as much weight as the people that didn't have anything, and one reason is that they were overestimated in terms of energy expenditure. So they allowed themselves to eat more, and therefore didn't lose as much weight.

Narrator: The formula for losing weight may look simple. Burn more calories than you consume. But actually measuring calories isn't, at least on your typical exercise machine.

Mikael Mattsson: The reason it's so difficult to get the calorie counts correct is that you're not really measuring the calorie counts. So to do that, you need to measure all the cells in your body and how much energy they are using. And obviously, you're not going to do that. So instead you, you're trying to use some other proxies for that. So you might measure the distance you're traveling or you might measure your heart rate or you might be measuring something else. The power output on the elliptical for example.

Narrator: According to Mattsson, it largely comes down to how well each machine calculates "work output" - which is a relatively reliable proxy for calories. And …

Mikael Mattsson: The more complex a machine is - a StairMaster and an elliptical is more complex than a bike - the less accurate the power output will be.

Narrator: For one study, researchers compared calorie readouts for ellipticals, stair climbers, stationary bikes, and treadmills. The calorie count on ellipticals was off by 42 percent. While the other equipment had a noticeable but smaller margin of error. Entering your weight before working out is a simple way to increase accuracy. That's because different bodies burn calories at different rates - even if they're doing the same activity. But even if you do, there's still plenty of room for error. For example, the machine might not be well calibrated.

Mikael Mattsson: The power output needs to be calibrated probably way more often than most of the normal gyms are doing today. If that treadmill is saying that you're doing 6 kilometers when you actually ran 5, then obviously the calorie count will be off. A same holds true for, for all the other equipment.

Narrator: And what about using heart rate monitors - are those any better? Not really. In a study published in 2017, Mattsson tested the calorie counters of several wrist-born monitors. The most accurate one was off by an average of 27%. And the least accurate? It was off by a whopping 93 percent, which can translate to hundreds of calories. And those heart rate monitors built into exercise machines at the gym might be even worse, he says.

That's not to say you should ignore calorie counters altogether. They can be helpful for measuring your progress - if you stick to the same machine at the gym. But unless you're riding a well-calibrated stationary bike, you're unlikely to get an accurate count.

EDITOR'S NOTE: This video was originally published in July 2019.