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, Dr. 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.

Dr. 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,” Dr. 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.