Is it true??

I always wear my hrm when I'm exercising to get the most accurate estimate (that's right, still an estimate). I tell my class participants what I burn, but making a general statement like "good job, you've all burned between x and y" is really false advertising. The fact is that it depends on if you are a man or woman, what your base fitness level is, how much excess weight you're carrying around, and the effort each participant puts into the class along with other factors. Also, just because you're all hot and bothered doesn't necessarily cut it as a measurement because there are enviornmental factors that play a huge roll in that too (which can also impact effort). I hope she's right, but I wouldn't count on it.

Calorie burn will vary with different people doing the same workout b/c different people will have a different age, weight, height, sex, fitness level, and effort. I would not trust a blanket estimate of 'everyone burns 800-900 in my class' - there are just too many variables.

I always wear a HRM when I workout - I have the Polar FT4 ($90 at Target, closer to $70 on Amazon) and I go by what it tells me. Like pp said, it is still an estimate, but it is a much better estimate than a blanket 'everybody burns' estimate. ;-). It is possible that you are burning that much, but it's also possible that you are burning more or less, either of which could throw your plan off a bit if you are eating your exercise calories.

If you don't eat your exercise calories or if you use a calculator that takes the frequency and intensity of your exercise into account (MFP does not, but calculators on other sites do) then I wouldn't worry about a perfectly accurate burn as much. Freedieting.com has a calculator that pre-adds exercise calories if you tell it how often you exercise. Only downside is that's not completely accurate either since it just asks how many times per week and whether it's intense.

You will never know unless you use a HRM - I also have the FT40 Polar - Best $ I spent for working out.. Its usually less than what the average on here tells you so... it helps a lot so you don't overeat.

It is just an estimate, and the more information you have to program into your watch, the more accurate it is going to be. If you are really interested in finding out exactly, I would suggest getting a Exercise Metabolic Rate Assessment done. I do them in St. Louis, however you can go to www.newleaffitness.com and put in your location to find the closest testing place available to you. By getting this assessment done you will be able to find out exactly how many calories (both from fat storage and carb. storage) that you burn at various heart rates. The reason this is accurate is because it measures your VO2 (Volume of Oxygen) consumed and your VCO2 (Volume of Carbon Dioxide) produced since you will be wearing an oxygen mask on while performing a 15 minute exercise assessment on a piece of cardio machine.