I’ve been doing Bikram and other forms of hot yoga for about 6 months and love the results. Never worried much about how many calories were burned… but I do admit to always being a bit curious, since there are so many different estimates out there.
I also run and have wanted a heart rate monitor for training purposes, just recently got one. The monitor I got also measures calories (it’s a high end model). Just for kicks I wore it to Bikram last night – I burned 721 calories (I’m 5’11” 136 lbs). It was FAR more than I was expecting. Then again, I work my butt off during Bikram, I don’t sit out any poses. I think the way you approach your practice would effect how many calories you are actually burning.
It wasn’t very comfortable doing yoga with that chest strap on, so I won’t be doing it again – but at least I have a little more fuel when people mention yoga as a “joke” when it comes to hard-core exercise! Burning 720 calories in 90 mins is no joke!