Calculating calories on a suunto sportswatch

Hi all,

I've had my suunto T3 for a while now and am very happy with it. There's one thing about it that I don't get though (and that's probably not restricted to suunto sportswatches): how are calories used during exercise calculated?

I'm pretty sure it's a function of average heartrate and duration, but there must be some more variables. I can think of hours of workout per week as proxy for level of fitness (which suunto calls activity class). Maybe gender?? Weight??? 

Does anybody know if there's a specific formula for this? I've had a go at it, but didn't manage... Am I missing a variable?

Thanks!!