Q: Is my heart-rate monitor accurately reading how many calories Im burning? -DURELL D., LAS VEGAS, NV

A: It probably has a margin of error of about 15%, says John P. Higgins, M.D., associate professor of medicine at the University of Texas Health Science Center. Factors such as height, weight, and age help predict calories burned based on common algorithms, but your fitness, gender, genetics, exercise duration, and body mass index also help determine calories burned, he adds. The most accurate models use both a chest strap and wrist monitor.