People exercise for different reasons, and different exercises fulfill those reasons with different degrees of success. If you want to lose weight, one of the best indicators of which exercise to choose is how many calories you burn with that exercise. Swimming, lifting weights and the elliptical machine are all common choices in the gym -- and each can help you burn calories at a different rate.
A calorie is a unit for measuring energy, the same way an inch is a unit for measuring distance. You take in calories when you eat food, and burn them via activity over the course of the day. It takes 3,500 calories worth of food or exercise to add or remove one pound of weight.
You can paddle around in the water and burn a few calories, but lap swimming is a legitimate and vigorous exercise. According to Harvard Health, a 155-pound person burns between 446 and 744 calories in an hour of swimming, depending on his pace and the stroke he uses.
Lifting weights is a resistance exercise, meaning it builds muscle strength and endurance more than it burns calories or provides a cardiovascular burn. Harvard Health reports that a light session of weightlifting will burn an estimated 224 calories per hour for a 155-pound person, while vigorous weightlifting will burn 446 calories in the same amount of time.
Elliptical trainers are a cross between a stationary bike and a treadmill, providing the workout of a jogging session with the reduced strain on leg joints you get from a cycling workout. A 155-pound person will burn about 670 calories in an hour-long session on an elliptical trainer.
Calorie burn is an art of approximation, not a precise science. The information given for any exercise is a best estimate based on aggregated statistics. Some factors that will cause your exact burn from a workout to vary include your body weight, level of conditioning, how long ago you ate, the air temperature and even your mood.
The difference between the calories you burn in a single session of one exercise or another is less than one-tenth the burn you need to lose one pound. This means that the calories you burn will only really matter if you can keep up a schedule of regular workouts. In general, the exercise you choose is less important than your commitment to a regular workout session.