How can 60 minutes on a treadmill burn 853 calories?

Like others have said, one's weight, age and intensity during the work-out are important factors.

http://www.runnersworld.com/tools/calories-burned-calculator

^I use this to get a vague idea since the treadmills are so very inaccurate.

It is impossible to say...way more information would be needed than you provided...which is also why data base calorie burns are notoriously inaccurate. There are just way too many variables involved to just go into a database and say I did X for 60 minutes and I weight Y....way more variables than that at play.

Also, if your rather unfit then your perceived level of effort is going to be skewed...because lack of fitness makes everything seem "vigorous" and intense. So people will say "vigorous effort" in a database not understanding that the database or calculator is assuming you're already at a relatively good level of aerobic fitness...so when you say "vigorous effort" for your 30 minute swim, the calculator is shooting back Michael Phelps type of calorie burns.

In general it is rather difficult to burn more than 10 calories per minute...that's working pretty hard. For the same level of effort you may burn a few more calories per minute if you're substantially overweight or a few calories less if you're lighter. When I was doing this, I seriously questioned anything greater than 10 calories per minute...for me that was equivalent to running about 6 MPH and I knew what that felt like...so if I didn't feel that level of effort with something else, I knew I wasn't burning that much or more than about 10 calories per minute.

So I plug in treadmill and I get stair-treadmill ergometer, general... 60 minutes gives 853 calories. That is a huge amount...could this be right?

Instead of plugging in a machine, type in the activity you did. Was it "run", "walk", "jog"? Pick the right activity and speed to get a better estimate, though still probably wont be right, but much better than what you did.

I don't know about you but I have never treadmill before, I have run, walked and jogged on a treadmill but no idea what exercise tread-milling would be (yes being sarcastic here)

Depends what you're doing. Like the others said, use a HRM. My elliptical at gome says I burn 800 cals in 45 minutes- the one at the gym says 460. My HRM generally says about 520-560 depending on my intensity that day. I go by the HRM (though I would LIKE to think 800 )

I somewhat agree, except that I would point out that treadmills are much more accurate than an elliptical as the equation for work performed (METs) is pretty standard and studied. The motion, etc on an elliptical makes it much harder to figure out the amount of work being performed. So in your case I would say HRM is more accurate than the elliptical machine, when it comes to treadmills, most of them will actually be better than a HRM due to the equation of acutal work done, vs. the HRM making assumptions on how much work was done.

If ur depending on the the cals burnt by the TM reading, make sure you use a mode where it asks you for your sex (M or F), age, and weight. Otherwise it assumes you are a 25 male weighing 150 lbs. Usually, the manual workouts on the TM will ask you for that info.

Instead of treadmill, look up walking. There are a lot of listings for different paces. Find the one that most closely matches what you did and go with that. MFP listings for calorie burn tend to overestimate so keep that in mind - plan to eat back somewhere between 50 and 75%.

In general it is rather difficult to burn more than 10 calories per minute...that's working pretty hard. For the same level of effort you may burn a few more calories per minute if you're substantially overweight or a few calories less if you're lighter. When I was doing this, I seriously questioned anything greater than 10 calories per minute...for me that was equivalent to running about 6 MPH and I knew what that felt like...so if I didn't feel that level of effort with something else, I knew I wasn't burning that much or more than about 10 calories per minute.

You deserve a VO2max test for so many reasons.

Because I'll bet you'd be shocked how much you actually burn per min.
Per my test, I'm burning 10 cal/min down below the recovery HR zone, which is 61% of tested HRmax. At top of aerobic HR zone 81% of HRmax, I'm burning 17.8 cal/min, and I can be there for a long while.

Now being fit just means I'm hitting that burn at lower HR, but still I'm betting most would be surprised it's not that hard to hit 15 cal/min.