As someone who has more than a glancing familiarity with the gym, the most frequent question I'm asked is "Are the calorie counts on cardio machines accurate?"
The short answer is, not really.
But you can make them a bit more accurate. If you just hop on and press "manual" or "quick start," you will get the default calorie count, which is generally set for a 150-pound 35-year-old male. If your are female, you will burn fewer calories. If you are older you will burn fewer calories. If you weigh more, you will burn more calories. The calorie readouts are based on equations, so if you input your own weight, age, and gender, the count will be a bit more accurate, but not totally accurate, because everyone's level of fitness is different. If I jump on a treadmill and run, I'm going to burn fewer calories than a novice runner because my body is trained for running. My movements are more efficient and my heart rate is going to be lower.
The best way however, to get an inaccurate calorie readout is to hold onto the handles of the treadmill or Stairmaster. I see people all the time just flat out draped over the stepmill, resting probably 25 percent of their bodyweight on it. This is wrong. Don't do it. For one thing, it burns fewer calories. For another, you could be using your core to stabilize you and thus working those muscles. And finally, it makes you feel tireder than if you were standing tall and striding proudly.
If you're serious about getting an accurate estimate of the calories you're burning, invest in a heart rate monitor. When you program it with your weight and age, it will figure out how any calories you're burning based on your actual heart rate.