Originally Posted by SSP
AFAIK, there's no standard formula for that. Each of the manufacturers use their own proprietary algorithms to estimate "metabolic equivalent (METS)" based on heart rate. They then take what they know about you (including gender, height, weight, etc.) and determine your basal metabolic rate. They then, presumably, multiple your basal rate by your METS to determine calories burned per hour.
FWIW, for road riding a good rule of thumb is 40 calories per mile.
That's what has always bugged me about my HRM, it doesn't have anywhere to enter age, height, weight - anything. Consequently, I take what it tells me with a huge grain of salt.