It may well be very motivating to watch the number of calories burned tick up throughout your workout as you slog away on the treadmill, elliptical machine or exercise bike, but you need to be careful and take those numbers with a pinch of salt.
Calorie counters on exercise machines are a useful guide, but there are a number of factors which make them unreliable:
Different people burn calories at different rates - Depending on your age, weight, fitness level and other genetic factors, people burn calories at different rates. Various exercise machines may allow you to input some of this data before you begin, which will help with the accuracy, but none can completely be tailored exactly to you, so they will never be fully accurate. The big factor in how many calories you burn is how much muscle mass you have.
You can use the machine in different ways - Small variances such as supporting a little of your weight on the treadmill by having your hands on the rails, adjusting the amount you use your arms vs your legs on the elliptical machine or how tightly you grip the heart rate monitor, can all alter the number of calories burned.
Some machines are more accurate than others - Treadmills are usually the most accurate for counting calories burned, as long as you input your weight and don't’ use the handrails, where as elliptical machines are the least accurate.
This doesn’t mean that you should ignore it completely though. It can still be a great tool for challenging yourself to beat your number from last time (try to use the same machine) and when you are pushing through the last few minutes of your workout it’s nice to see the calories burned number ticking up, even if you know it may not be 100% accurate!
Unfortunately, there is no practical way of measuring how many calories you burned during a workout. You should push yourself during your workouts, and then be mindful when you are refuelling your body.
If you are trying to reduce your body-fat, remember that although cardiovascular exercise does burn calories, it is your "muscles" that do the burning, so a much more effective way of burning calories is to increase the amount of muscle you have on your body.
The best way of increasing muscle is through resistance exercise (such as weight training). By having more muscle on your body, you will continue to burn more calories even when at rest!
Creating a calorie deficit is important when you are trying to slim-down, but remember that the aim should be to USE more calories and convert them into muscle!