Here are some formulas I found for calculating calories:
Calories (kCal) = 0.28 * (weight in pounds) * (distance in miles)
Calories (kCal) = 0.174 * (weight in pounds) * (distance in kilometers)
Calories (kCal) = 0.653 * (weight in pounds) * (distance in miles)
Calories (kCal) = 0.406 * (weight in pounds) * (distance in kilometers)
Forget it... technique makes too much of a difference.
The running one has a chance of being somewhat accurate, there are fewer variables in running. But cycling can be done on good or bad bikes, on flats or on hills, with head winds or tail winds, on roads or off... etc. All the factors in cycling have big impacts on calorie consumption.
But it will give you a ballpark that you can fudge for all those factors. Add a few calories if you are on a 50lb mountain bike, drop a few if you've got a Cervelo, that kind of thing.
(As an aside, units of measurement are funny, especially in a whacky country like Canada. We talk about our weight in pounds, but our distances in kilometers. Thus my hybrid Imperial/Metric versions of the formulas... officially we're metric, but we're too close to the U.S. to avoid their influence! Sorry to the Europeans for not providing an all-metric version... and for the baseball analogy in the preceding paragraph...)