Your fitness tracker, treadmill, or stationary bike can estimate the calories you burn. Just don’t put too much stock in those numbers.

“Trying to determine how many calories somebody is burning is not easy,” says John Porcari, professor of exercise and sport science at the University of Wisconsin−La Crosse. Porcari writes the calorie-burn equations for some companies that make exercise machines.

Two key factors that affect calorie burn

“First, the more work you do, the more calories you burn,” notes Porcari. (Work is the amount of effort you exert and for how long.)

“Second, a bigger person burns more calories because they have more body to move. If a machine doesn’t consider body weight, you have to question how accurate it is.”

And machines also can’t control for other factors. For example:

Fitness level. “Beginners often have more extraneous movements than seasoned exercisers, so they burn more calories doing the same activity,” says Porcari.

Handrails. If you hold on to the rails on a treadmill or stair climber, you burn fewer calories.

Walking or jogging? You burn more calories per minute when jogging than walking, but a treadmill can’t tell if you’re walking fast or jogging slowly.

Why ellipticals are the trickiest

“The machine doesn’t know if you’re using the arm levers, and you’ll do less work for leg-only than for leg-and-arm exercises,” says Porcari. “And ellipticals have a fixed stride length, so if I’m really short, I have to do more work than someone who is taller.”

How far off are most machines? “They may be as much as 20 to 30 percent high or low,” says Porcari. So if you burned 300 calories, your bike may say anywhere from 210 to 390 calories.

What about Fitbits, Garmins, Apple Watches, and other fitness trackers?

“Their calorie estimates are often worthless,” says Porcari. “Some try to quantify how much work you’re doing based on your heart rate, your speed, and the number of steps you take. But it’s difficult to get an accurate read.”

In one study, Porcari used a portable metabolic device to get an accurate read on the number of calories burned by 20 volunteers as they walked, ran, did elliptical exercise, and performed basketball drills. He compared that to the numbers from five fitness trackers that the exercisers wore.

The trackers were off by 13 to 60 percent. “They were okay for walking,” says Porcari. “They weren’t very good for running or on the elliptical, and they were absolutely terrible on the basketball drills. When you get into more complicated movement patterns, the equations really fall apart.”

Don’t throw out your Fitbit, though

“The absolute number you get might not be accurate,” explains Porcari. “But you can use them as a rough guide to make workouts harder or longer.”

The bottom line: Don’t take calorie-burning numbers literally.

Photo: Danai/