I have one that I use it is a Polar and I used to be really in shape and could run two miles at a time, now i'm overweight and trying to get back on track. But I used mine and was on the treadmill for 20 minutes and I (embarrassed enough) after twenty minutes put up a nice sweat, and it said i burned 442 calories.  Think this is accurate? Probably is since i'm so out of shape....and a bit overweight.

Amanda,

Does your treadmill have a heart monitor already associated with it? The reason I ask is because, I also have another heart monitor that i decided to use (which did not come with the treadmill). and I did a run. It showed a much higher heart rate and calories burned. Next day I used the Treadmills heart monitor and that was more "realistic" for the workout I was doing. Electrical interferrence could be the cause. Not sure...

But, if you were moving at a good pace for those 20 minutes and had some incline to your workout... I bet you could get close to that...

I would try another workout with the monitor, at a track or piece of measured road..and see if that has any flux in the reading. Also, since it is polar, don't you need to send it to them to have the battery changed? My gf had one a few years ago...she changed it herself...totally hosed it...The battery might need to be changed... Good luck!!! and Keep it up!!!

Just wanted to jump in since I am also overweight and have been running for the last year.  I use a heart rate monitor but that doesn't necessarily determine how many calories you are burning - just another stat to see your progress.  It is absolutely true though that the more you weigh, the more you burn when you exercise.  I found a great calorie burner tool that I figured I'd share.  Good luck!!

http://www.runtheplanet.com/resources/tools/calculators/caloriecounter.asp

This site has a great little chart of calories burned in one hour of exercise, for people weighing 130, 155 and 190 pounds.  Everyone's different, but you'd probably have to run 8 or 9 miles per hour to burn that many calories in 20 minutes.

http://www.nutristrategy.com/activitylist4.htm