It depends on three things, Your Weight, Your Age, and The Resistence. Unfortunately most exercise bikes do not get it perfecetly right. Some of the the better bikes(Life Fitness) do have a pretty good equation but they are not 100% accurate. The only way to get an accurate count is to invest in a good heart monitor(I suggest the Polar F11). However, there is one thing you didn’t say. Are you trying to lose fat or better your cardio vascular system? The heart rate at which you burn fat(most efficiently not that you don’t burn fat at higher heart rates) and where you give you cardio system a work out varies greatly. For a 30 year old the fat burn rate is around 115-130 and cardio is 135-170. This also depneds on your althletic level(something the monitor can help you to determine). So in the end there is no real set number for any exercise that fit all people. Personally I go by the time(on the exercise), the resistence, and my heartrate, 30 min, on resistence 3 @130bpm = weight loss for me.

Me personally? Average woman, small stature, experienced runner ca. 400 calories/hour.

it really depends on the intensity at which you do these things….