Sarah, that is a hard question to answer. I know that at one of the gym's I used to go to, halfway through an aerobic class, you would do a pulse check. You would count the beats over certain period of time (I think it was 10 or 20 seconds) and then you check the chart to see if your pulse is in the fat burning zone. I don't use the machines that much so I don't know how many calories I burn. My own rule of thumb is if I can speak without effort around the half way mark, I need to push myself. If I am having trouble speaking (or even breathing
) then I need to ramp my excercise down. You learn to pay attention to how your body is feeling.