why the huge difference?

I can't answer why there is such a huge difference, but I would be scared to trust either one. If you really want to find out how many calories you're burning, you'd be better off getting a heart rate monitor. It would be much more accurate than MFP and the machine. When I wore a heart rate monitor, it showed that I burned less than the machines told me. Now I use a bodybugg (sensor attached to my arm, it's supposed to be 93% accurate) and what it says I burn is way less than what MFP says. The devices that you wear will be more accurate because they've got more info to base the calorie estimate off of. I don't know how MFP comes up with a measurement, but workout equipment only goes off of weight, and I've heard it bases calories burned off of being a male at a certain height that I can't remember.

Who told you a HRMis not accurate?

And different things give different readings because they're using different formulas

Who told you a HRMis not accurate?

And different things give different readings because they're using different formulas

My p/t says they aren't always accurate (which I don't believe) so I'm with you saying that they are pretty accurate.

I wouldn't trust the machines or MFP. They could be either highly over estimating or under estimating. Go get a HRM.

The accuracy of an HRM depends on the manufacturer. One thing you can do is choose the estimate that seems most relevant to you and monitor your progress... If you find yourself progressing faster (on the long term) than you should, you are either underestimating burn or over estimating calories (or both)... If you find yourself progressing slower (or not at all or going backward) then most likely you are overestimating burn and/or underestimating calories. At best, this is an inexact science... Too many variations. But it is so much better than not keeping up with it at all... It gets us in the ballpark. Best wishes on your journey.

I messed up the reader for my HRM this year (grrr), but when I would wear it with my stationary bike it would say I was torching like 650 calories in 40 minutes on my bike at a resistance of 1 and riding roughly 14mph. I weighed just a hair over 200lbs. MFP estimates a litte less than that (~500), and my bike way less at around 200 calories.

Since breaking it I had to choose between MFP's estimates and my bike's estimates (for example, today's ride according to my bike was about 188 calories burned, and 390 or so on MFP). I chose to go with MFP and have been losing for about 6 weeks.

Does your bike allow you to input your age, weight, gender, height?

I ride my recumbent bike pretty hard (for me) at level 4 resistance for a half hour. My bike says I've burned 125 calories but MFP says I've burned over 300. Why such a huge difference?

database calorie burns are wildly inaccurate due to about a million variables. I might do 30 minutes of swimming and you might do 30 minutes of swimming...but my intensity level may be more or less than your intensity level for example...thus I would burn more or less calories. I think most databases assume you're working at some elite level of intensity because they always seem to wildly overstate burn...which is one of the big reasons I think people say this doesn't work for them...they're vastly overestimating calorie burn.

Truth be told, people burn far less calories than they think they do with exercise. It's one of the big reasons why it is far more efficient to build you calorie deficit into your diet rather than trying to build a deficit with exercise.

As far as estimations go, machine > data base...HRM > machine > database...but it's all still estimation. I think Polar models were tested to be about 70% accurate for calorie burn when used during an aerobic event for an average person of average height and weight.

If you're running or walking you can assume about 100 calories burned per mile give or take...I'm not sure what it is for cycling or other cardio events.

Who told you a HRMis not accurate?

And different things give different readings because they're using different formulas

I've read that a HRM is not accurate because your heart rate is only one of many factor that goes into determining how your body is burning calories. It uses the heart rate along with you age, weight, and sex and plugs them into an algorithm. I have an heart rate monitor and I use it only to see how my heart is responding to the stress of exercise and for nothing else.

I ignore the calorie counters on machines because they are so unreliable.

I heard a HRM is not accurate either. So I have no idea what to trust and i think it is hard to know exactly how many calories you are really burning

Only the cheap ones aren't accurate. like Timex sucks. Get a Polar (ft60 or similar).

I've always had Polars. I use them only to check my heart rate. Despite what the companies say, these tools can't do everything. If you really want to know, get a vo2 max test, which will be more accurate.