After my run today, I was stretching near the group exercise room at my gym. I noticed a couple of signs advertising an aerobics class and a weight lifting class. On those signs were estimations of calories that can be burned in those classes. I know there is no way that I would burn the amount of calories listed and many other people wouldn't burn that many either. I also can't help but laugh when I hear instructors yell out that we just burned 1000 calories in a class that lasted 45 minutes.
As stated, those calorie counts are just an estimation, and most likely an estimation on the very high end. "Calories burned" are typically calculated based on an "average weight" woman or man who is working to the fullest capacity in a class. Sorry, no breaks or lower impact options! These numbers not only do not take your individual weight into account, but they also do not take your gender, lean mass versus fat mass, fitness level, and age into account.
Wearing a heart rate monitor that takes your age, gender, and weight into account would be the best way to estimate your calories burned in any given activity. Granted, it will not be perfect either, but it will be a lot more accurate than the numbers any machine or instructor gives you at the gym.
This is really an important thing to know because many people overestimate their activity without the help of inflated calorie burns. When you start hearing that you're burning massive amounts of calories (even if you aren't), it is human nature to start eating a little more than you need to.
Ultimately, the best thing you can do is to exercise, eat when you are hungry, and stop eating when you are satisfied. Try not to get tied up in numbers that are inaccurate and unhelpful.