I was just thinking about how accurate they are, but I'm a little confused.

The only input variable is the users heart rate. There are also pre-entered/constant values like age, resting HR, max HR, and weight (not sure how they are used for the calculation).

Ok, here is what confuses me. Let's say in reality I am burning 700 calories in one hour, with an average HR of 160 BPM. After some time, I become more powerful, none of the constant values change (age, weight, etc.), and I am burning, say, 1000 calories in one hour, with the same average HR (160).

If what I said above is correct, then a HRM has no accurate method of calculating the calories burned. Am I right?

Also, does anyone know a mathematical formula for calculating calories burned, based on HR?

Thanks

Most HRM ask you to input height / weight and age..

From that and effort during an exercise it gives you calories burned.. I always felt that they estimate a little on the high side..

The Polar 700 series HRMs ask you to enter your personal VO2max figure, which scales linearly with power and calorie consumption. More oxygen consumed = more calories consumed.