Companies including Nike, Polar Electro and Timex that offer such gadgets as pedometers and heart monitors to help people fine-tune their workouts have started adding calorie counters and special Web sites.
Experts say the calorie-counting gadgets are far from 100 percent accurate but better than the guesswork most people use.
"It seems simple, if you expend enough calories, as many as you consume, you won't gain weight. But it's not easy for the average person to know--How many calories I'm using in that workout, that aerobic dance class I just did," said Janet Rankin of the Institute for Biomedical and Public Health Sciences at Virginia Tech.
"In general people are going to overestimate. I think these kind of tools can provide a little bit of a reality check and serve as a marker for improvement," she said.
The arm bands, chest straps and wrist bands try to offer a glimpse of information that can typically be seen only in a lab. The calorie counters are based on two methods researchers use: how much oxygen is being used and how much heat is being produced.