Calorie Burn Bicycling With a Trailer

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Some quick background....... I'm a 6'0" male and weigh 260 lbs. I ride a couple times a week. One or two short rides with my daughter, and then about 22 miles on Saturdays. Based on reading some threads here, I estimate my calorie burn from cycling at 25 calories per mile.

First, I wanted to know if people thought that was a reasonable estimate. I have a hybrid Cannondale bike, ride on one of our local paved trails, ride about 13 mph.

Also, I got the trailer attached tonight and took both girls out. My two girls combined weigh approximately 100 lbs (65/35). How many extra calories per mile should I expect to burn with one or both of them? I'm not quite sure what my pace was, it was just a quick ride to get them out and let them experience the trailer. Is there a formula where I can plug in some variables such as weight, distance, speed and get an approximate burn? I'll have to use mapmyride for my pace.

Any thoughts? Thanks in advance!