Studies show that different people burn vastly different numbers of calories even when they do exactly the same thing while exercising. That’s bad news for folks whose instructors have been saying, “You’re burning 900 calories a class!”

MCT / NURI VALLBONA

The spinning class at our local gym was winding down. People were wiping off their bikes, gathering their towels and water bottles, and walking out the door when a woman shouted to the instructor, “How many calories did we burn?”

“About 900,” the instructor replied.

My husband and I rolled our eyes in disbelief. We looked around the room. Most people had hardly broken a sweat. I did a quick calculation in my head.

We were cycling for 45 minutes. Suppose someone was running and that the rule of thumb, 100 calories a mile, was correct.

To burn 900 calories, we would have had to work as hard as someone who ran a five-minute mile for the entire distance of nine miles.

Exercise physiologists say there is little in the world of exercise as wildly exaggerated as people’s estimates of the number of calories they burn.

Despite the displays on machines at gyms, with their precise-looking calorie counts, and despite the official-looking published charts of exercise and calories, it can be all but impossible to accurately estimate the number of calories you burn.

You can use your heart rate to gauge your effort, and from that you can plan routines that are as challenging as you want. But, researchers say, heart rate does not translate easily into calories. And you may be in for a rude surprise if you try to count the calories you think you used during exercise and then reward yourself with extra food.

One reason for the calorie-count skepticism is that two individuals of the same age, gender, height, weight and even the same level of fitness can burn a different amount of calories at the same level of exertion.

Claude Bouchard, an obesity and exercise researcher who directs the Pennington Biomedical Research Center in Baton Rouge, La., found that if, for example, the average number of calories burned with an exercise is 100, individuals will burn anywhere from 70 to 130 calories.

Part of that is genetic and part is familiarity with the exercise. The more familiar you are with an exercise, the fewer calories you use at the same level of effort, he found in a research study. Subjects rode stationary bicycles six days a week for 12 weeks. They ended up burning 10 percent fewer calories at a given level of effort after their training. The reason, he said, is that people perform an exercise more efficiently as they become more accustomed to it.

There also is a seldom mentioned complication in calculating calories burned during exercise: You should subtract off the number of calories you would be using if you did nothing. Almost no one does that, Bouchard said. But for moderate exercise, the type most people do, subtracting the resting metabolic rate can eliminate as much as 30 percent of the calories you think you used, he added.

Resting metabolic rates, though, differ from individual to individual and also differ depending on age, gender, body mass, body composition and level of fitness, so guessing at your resting rate also is fraught with error.

Even if you wanted to get a rough estimate of the calories an average person your size might burn at the gym, you might not want to trust the displays on cardio machines, with the possible exception of treadmills, said William Haskell, an exercise physiologist at Stanford. And with treadmills, the calories are not accurate if you hang on the bars.

Haskell once studied people using treadmills. Hanging onto the rails reduced the number of calories burned by 40 to 50 percent. The same thing happened with stair-climbing machines.

“I’ve seen people hanging on stair climbers who think they are doing 1,200 calories an hour,” Haskell said. “They probably are doing 600 calories an hour.”

As for the calorie counts on machines like stationary bicycles and elliptical cross trainers and stair climbers, all bets are off, researchers said.

A major problem is that the machines get out of calibration. “They drift in speed and grade,” Haskell said. “If you go from one machine to another, it is obvious that at the same setting you are working much harder on one and much less on the next.”

Another is that the companies use their own formulas to calculate what an average person of a given size would burn at a given level of intensity. And those formulas may vary by company as well as by machine.

My husband and I will never forget a mathematician at a meeting we attended when we were graduate students. He proudly announced that he could eat a piece of pie because he had just run a quarter-mile on the track. (At 100 calories a mile, he might have burned 25 calories running. A piece of pie could easily contain 400 calories.)

Bouchard said he regularly saw people at his gym who boasted about fantastic numbers of calories they believed they burned. Usually, he just bites his tongue.

“I don’t want to discourage them,” he said.

Sometimes, though, with people he knows well, he tries to tell them that the counts are way too high, and why.

“They look at me in disbelief,” Bouchard said. ‘There is some error, there is some play’

Jim Zahniser, a spokesman for Precor, cautions that the company’s exercise machines are not supposed to give the exact number of calories for you. Instead, they are estimates of the calories an average person would burn at that work level. And, the company explains on its Web site, there is no easy way to know whether you are that average person.

“Unfortunately, there are as many answers to that question as there are people,” Precor said on its Web site. As for calorie charts, which tell how many calories a person burns when running a distance at a rate of a 10-minute mile, for example, or walking at a rate of a 15-minute mile or riding a bike at 15 miles an hour, the same difficulties apply.

“If any company told you the calorie counts on their machines would be dead on, they’re lying to you,” said Mike Armstrong, a spokesman for Nautilus. “There is some error, there is some play.” He advises that you look at the calorie count, “with a grain of salt.”

Christa Dickey, a spokeswoman for the American College of Sports Medicine, which publishes such a chart, cautions that figuring out calories “is always a bit of a challenge since you can really only assign estimates.”