To be accurate...if you're at a gym, ask a personal trainer to put your numbers into the equation to determine energy expenditure. The tredmill should list METS for you.
It's a longer formula than I care to type out at the moment but it's basically you take speed and fractional grade and add it to 3.5 and then you'll come out with these numbers to multiply to give you how many ml per kg per minute. You then plug in bodyweight in kg, etc and come out with how many kcals per minute that you're expending.