I have a plain heart rate monitor which is very accurate, but any thing that calculates calories burned is going to be an estimate. Anything taking your weight and heart rate into consideration will be fairly accurate, but I don't trust the HR monitors on most machines either. They tend to register way high or way low compared to my monitor, so I'd definitely recommend getting your own if you're interested in keeping track. Polar is a pretty reliable brand, but there are a lot of different kinds out there. Good luck!