How do you guestimate how many calories??

I use a Polar Heart Rate monitor and I swear, it has boosted my weight loss a LOT! I bought the cheapest one I could find (FT4) and it has worked great for the last year! I do bootcamp workouts and never knew how many calories I was burning. It felt like 1000, but really, most workouts were only about 400-500. It definitely makes me work harder when I check it throughout class. I refuse to leave class without burning 500 now, and sometimes that means me staying a few minutes extra to do some jumping jacks with a friend.It's hard to get a general idea on calories. Your weight and heart rate have everything to do with that calculation. Invest $50 in a heart rate monitor, and I PROMISE you won't be disappointed! :o)