Guesstimating calories for exercise

Mr_Boy

Is there a simple formula one can use for cardio exercise? I was wondering if it really boils down to heart-rate for a given person, i.e. if I run at 140bpm would I burn calories as fast as swimming or cycling at 140bpm?

I've been playing Grand Slam Tennis on the Wii and it genuinely constitutes exercise I think, based on the test "does it make you sweat?" The game even has a calorie-tracking function but I don't trust it since it didn't ask for my weight, age, etc. I'd like to figure out a ballpark figure to use for an hour playing.