If you get your cardio in on a machine like the treadmill or elliptical, you probably watch the calorie counter tick away the calories you are burning with satisfaction. It may motivate you or may justify why you are on the miserable thing. Unfortunately, these counters are not all that accurate. It has been estimated that they can be off by 30% in either direction.
If you do not fill in your own personal stats, machines are programmed to assume you weigh 150lbs and base the count on that.
Machines do not take into account important factors such as gender, height, fitness level, oxygen consumption and heart rate (some do measure heart rate but many don’t or are not accurate).
Don’t fret. All of your hard work is not lost. What I suggest is wearing a heart rate monitor that also tracks calories burned. These typically ask for gender, age, height, weight and some ask for fitness level. They keep track of your heart rate and measure calorie burn based on the factors above. OR just wear a heart rate monitor and do intervals, allowing your heart rate to go up and down. Science shows that interval training of this nature produces a high calorie burn. You may not have an accurate indicator of # of calories burned but you will be sure to get the best bang for your buck!