Three factors determine the amount of calories burned during exercise: weight, duration and activity. According to WebMD's Exercise Calculator, a person who weighs 150 pounds burns approximately 341 calories cycling on a stationary bike and 273 calories riding a bike outdoors for 60 minutes.
Sex, age and body type are factors that further determine the number of calories burned during an activity. Men burn more calories than women do because they tend to have less body fat, and young people burn more calories than older people do for the same reason. People who have more muscle than fat burn more calories than those who have more fat than muscle.