Here are links to a few of bicycling calorie calculators...

How Many Calories Do You Burn Cycling? | Bicycling Magazine

Calorie Calculator - MapMyRide

Calculate Calories Burned Biking

I'm guessing that these and similar calculators may all be based on some long-past algorithm that gets repeated without question as time goes on. My first-hand experience suggests that Machka's idea of 400-500 calories an hour is closer to "reality" (whatever that is!). In my own case as a Clydesdale rider (6'5", 215 lbs), keeping up with the faster group on my club rides is a serious workout, so I believe that 500 calories per hour would be a good working estimate. Maybe another good option is to use the number from the Garmin edge but reduce by 60%.

Thanks for the comments and ideas!