We’ve all been there: After a hard-core tryst with the stair-climber or elliptical machine, you step off feeling virtuous when you see how many calories you just burned. Before you give yourself a high-five in the mirror or treat yourself to a 500-calorie scone at your favorite coffee joint, there’s something you should know: “The readout information on exercise machines can be off by as much as 20 to 30 percent — giving you numbers that are 20 to 30 percent higher than the calories you actually burned,” notes Michele Olson, a fellow of the American College of Sports Medicine, or ACSM, and a professor of exercise physiology at Auburn University–Montgomery in Alabama. “Over a couple of weeks, that [overestimation] can add up to thousands of calories, and it can really throw you off if you’re using it to decide what to eat and you’re trying to lose weight.”

Granted, some of the feedback you get from exercise machines — such as heart rate, distance covered and pace — is pretty accurate, says exercise physiologist Richard Cotton, national director of certification at the ACSM. But the calorie count is another story. And some exercise machines are more reliable than others in this respect. In a 2010 study at the University of California–San Francisco’s Human Performance Center, researchers evaluated the accuracy of four different cardio machines’ calorie counters, compared to a VO2 analyzer that assessed an exerciser’s calorie expenditure during a workout by tracking his or her breathing patterns. What they found is that all the machines tend to err on the high side, and some more than others: Stationary bicycles overestimated calories burned by 7 percent; stair-climbers did by 12 percent; treadmills by 13 percent; and elliptical machines by a whopping 42 percent.

There are several possible explanations for these inaccuracies, experts say. For starters, each manufacturer uses its own algorithm to calculate calorie expenditure, and your personal information may or may not jibe with that proprietary formula. “We respond to exercise in a very individual fashion that may be different from Joe or Jane Average,” says exercise physiologist Cedric Bryant, chief science officer at the American Council on Exercise. The calorie 411 on the machine “isn’t necessarily going to be accurate for you,” he adds, “because few of us match that average profile the information is based on.”

After all, the rate at which someone burns calories varies based on weight, age, gender and fitness level (someone who is less fit will expend more energy during the same workout as someone who is fitter, Bryant says). While some cardio machines allow you to enter your weight and age, few ask for all four variables, and some don’t ask for any of that information at all. “If the device doesn’t ask your body weight, it automatically puts in a preset weight that you may or may not be close to,” Bryant says. Among those that use a reference body weight, it’s often based on 70 kilograms (roughly 150 pounds), so if you weigh more or less than that, the feedback will be skewed in that direction.

Then, there’s the issue of wear and tear on the machines. On treadmills and other cardio machines, belts tend to slip as they age, which means the resistance that’s provided may not be what it once was, Cotton notes. The calorie-counting software can’t account for those age-related changes, which can result in miscalculations as far as calorie expenditure goes.

Sometimes, however, calorie-counting inaccuracies stem largely from operator error, especially regarding form and technique. Mistake No. 1: leaning heavily on the handrails on a stair-climber or elliptical machine. “When they’re leaning on the rails or propped up on their forearms [on the machine], people aren’t doing the work with all their body weight involved,” Olson explains. If you’re offloading a significant portion of your weight, the machine can’t detect that, which means you won’t actually burn as many calories as the readout indicates.

Similarly, if you put yourself on a particular grade on the elliptical machine or the treadmill, “you can negate the [calorie-burning] effects of the grade by leaning intensely on the handrails,” Cotton points out; again, the machine wouldn’t account for this. And if you’re using the elliptical machine and “you’re being passive about using your upper body, you’ll end up with an overestimation” of calories burned, Cotton says, because the machine wouldn’t register that, either.

The take-home message: Don’t put too much stock in the specific calorie readouts you get from a machine after a workout. And “don’t let the feedback guide your eating choices,” Cotton says. (Hey, it happens: A 2014 study from Germany found that people tend to eat more after performing a so-called ” fat-burning” exercise session than they did after an “endurance” session.)

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Do Exercise Machines Lie? What You Need to Know About the Feedback You Get originally appeared on usnews.com