Is my HRM working right?

Well then I decided to see what I burned at work. I am a Emergency Room nurse so I figured it'd probably be higher.... but its sitting at 8hrs 7mins... and it says I've burned 4,381 calories.

Is my HRM messed up or is that possible? I mean I am 6'2 and 290lbs(approx) so i am a big guy... but is that a reasonable number?

Well HRMs aren't made for all day wear. So it would be completely inaccurate - not messed up, it's just not what an HRM does. HRM is to track steady raised heart rate. For all day wear you need a FitBit or similar type device.

Likely not.

Use this to test against, more accurate than HRM.

And actually, way more accurate then them. The formula for getting calorie burn from HR is vendor secret, and Sportline ain't got the money to license or do their own studies.

See how fast you'd have to run or walk to hit that calorie burn. And if you think you could even do that.

http://www.exrx.net/Calculators/WalkRunMETs.html

Gross option is what HRM, or database table like MFP would be reporting. Net option is what you would eat back on MFP after logging it. So compare HRM to Gross, eatback NET to use MFP correctly.

Can do more thorough test too.
http://www.myfitnesspal.com/topics/show/774337-how-to-test-hrm-for-how-accurate-calorie-burn-is

And yes, even the bad formula's for calorie burn are going to only be for the exercise range, aerobic specifically, and steady-state with HR the same 2-4 min.
So no below exercise is valid, no anaerobic like sprints or lifting, and no non-steady state like lifting or much.

You may be able to adjust some of the HRM stats to get more accurate.

What did it ask for in personal stats?

So forgetting the calorie burn figures since they'll likely never be close, pay attention to the the HRM does right, monitor HR, and note the avg HR reached if you can do several treadmill tests.
Note your pace and speed.

As you get more fit, HR should come down.

If you do another cardio workout, if you hit that same HR, you burned the same calories basically per minute.

So it'll still be very useful, just gotta record some stats now.

But it'll always be inflated for the P90X, which is strength training, anaerobic, and non-steady state to the max.

Chest strap does indeed make it EKG accurate - for measuring HR.

There is no measuring of calorie burns, that's all calculated from HR.
And with those few stats, a lot of assumptions are made.
I'm betting it's figuring out BMI with height and weight, and then a table with age to determine how good or bad your BMI is. No gender though, so rougher than Polar method.
From that, you can estimate roughly a VO2max - that's what's used for actually determining calorie burn.
Age is also used to calculate HRmax figure - 220 minus age.

Shoot, even the cheaper Polars make many assumptions in their calc's for calories, so your model isn't alone. Though that exaggeration does seem to beat the Timex models.
Polar on the cheaper models even uses that same BMI method, as there is a study and formula in public domain for that calculation, though I'm sure Polar tweaks it. Or may not those cheap ......

FitBit is using step based calculations with your weight and estimating pace and basically using the same formula that calorie burn link is using. Weight, pace, and time = calorie burn.

But it's still not good for non-step based stuff like lifting.

I'll think you'll get more from the HRM though, used for monitoring HR.

If you can do that treadmill test after you see what your avgHR is for a P90X session, and get up to a flat speed that reaches that same avg HR for 20 min. Decide if that feels like average intensity of the P90X too.
Then use that calc to see how many calories NET you burn doing that speed at current weight. Same time as video session, or per min or per hr, whatever allows you to get the math you need.

Then as you get more fit and HR lowers for same level of effort, do another test monthly. HR will lower for doing the same amount of work.
That's where the HRM will be interesting, see how you improve in that way too.