I was asking myself this question today and decided to put Google to the test and see what Google may be able to direct me to. I found this site. http://www.shapeup.org/interactive/rmr1.php . You plug in your height weight and age, and it will tell you how many calories you burn by just being yourself.
This is helpful since the way you lose weight is by burning more calories than you consume. It really is that simple. On some websites they were suggesting working out to the point of burning 1,000 calories when you eat 1,000 calories. Since going to the gym recently, I knew this was absurd. That is a LOT of working out in one day. So learning how many calories I burn by doing nothing is nice to know.
This calculator told me I burn about 1,800 calories by sitting around and doing diddly squat. Since I am only eating 1,500 calories now and burning at least 300 additional calories at the gym each day this means I am burning 600 more calories than I am consuming every day.
Now a basic number on weight loss is that you need to burn 3,500 calories to lose one pound. This should put me at losing about 1.5 lbs each week. So if I keep this up for 33 weeks I should be at my 150lb goal. That would be just before June of next year. It is hard for me to imagine right now that I could be at my high school weight by next summer, but it is very encouraging that the effort I am putting each day builds up for real results.