So if you want to lose weight, one of the most important things you need to know is how many calories a day you actually burn.
Over the years there have been many methods and equations used to make this determination. The picture in this post is an old standard called a, "Douglas Bag". Basically it allows docs to figure out how much oxygen you've burned and extrapolate from there how many calories you've burned.
Newer methods include such things as indirect calorimeters and some even fancier devices.
While there are some places that have indirect calorimeters (like us at the Bariatric Medical Institute), at least 90% of the time the equations are pretty accurate.
The best equations I've found are located here.
Once you know how many calories you burn then if you simply keep track of calories using a food diary or an online service, so long as you eat less than you burn, you'll lose weight.
The trick is, you've got to like the way you're living - that's where keeping track of calories is so helpful. By having an idea of how many calories you burn and keeping track of how many calories you have it allows you to answer a very easy question, "Is it worth the calories" when you're considering having something to eat. The other thing to remember of course is that sometimes, it ABSOLUTELY is worth the calories. Food plays a role in comfort, celebration and just like that decisions. If you lose the ability to choose less than perfectly from time to time, well then you're on a diet, and guess what, diets fail over 98% of the time.