Let's say you run for half an hour, and the treadmill reports that you burned 350 calories. If you're counting calories to lose weight, then you should be aware that that 350 calories might not be what it seems.
The treadmill probably calculates the total calories you burned rather than the net calories burned.
The total calories burned includes not just the extra calories you burned by performing a cardiovascular activity, but also the calories you would have burned anyway if you'd been sitting back on the couch watching television.
Net calories burned, on the other hand, is what you burned on top of what you would have burned anyway.
How many calories would you have burned in that 30 minutes, if you'd been sedentary? To get an accurate count, you'd have to breathe into a machine that calculates the calories you burn based on your respiration.
However, there are several formulas that can give you a pretty good estimate of the number of calories your body burns just to keep itself alive day after day. The more accurate of these take into account your weight, height, age, and sex.
For a more general estimate, you can figure that a sedentary lifestyle requires about 12 calories per pound per day. If you weigh 150 pounds, then that's 1800 calories. At that rate, you burn 37 calories every 30 minutes.
To figure the net calories you burned on the treadmill, you have to subtract those 37 calories from the 350 calories you burned in total. This means you burned 313 calories in addition to what you would have burned anyway doing nothing.