The idea of "calories in/calories out" is a good place to start... but you have to remember that every body is different, and that once you get into the obese category, your body will probably not deal with things in a straightforward manner. I know for me, my body is insulin resistant and I have PCOS, so I need fewer calories than most of those calorie counters tell me if I want to lose consistently.
What I did was go to a bunch of the counters, and instead of putting in my current weight, put in my goal weight. That gave me an idea of calories to shoot for. I also put my food into a calorie tracking program (I use Fitday, below) for a couple weeks, to get a realistic view of how much I was actually eating. It shocked me to find out that it is NOT hard for me to eat 2500-3000 calories a day when I'm not paying attention.
So first off, I limited my calories to 2000-2200. Most of my friends who are on doctor-approved diets aim for 1800-2000, so I thought that was a good place to start. And if it's just a matter of calories in/calories out, I would have to lose since I was restricting my calories, right? Well, yes, but much more slowly than I wanted, like a pound every couple of weeks. But I was learning better habits, so I wasn't too upset.
Then when I was pretty happy with some of my better eating habits -- like measureing my food and getting more fruit and veggie servings -- I lowered my target again. I have read that one shouldn't go lower than 1200 calories a day. And in truth, when I go low I end up suffering for it from hunger and headaches and *****iness and such. My body needs more fuel than that.
So I aim for 1500-1800 calories a day. When I eat closer to 1500, I tend to lose 1-1.5 pounds a week. When I eat closer to 1800, I tend to lose .5-1 pounds a week. This is looking at month-long trends, though, because my weight fluctuates a lot. Even when I am eating around 1500 consistently, there are weeks when I "gain". I know it's just water fluctuation, so it doesn't bother me. Over time, I've been pretty consistent in losing about a pound a week in this range of calories, with one "free" day a week where I don't count them at all.
I also find that if I limit sugar and white carbs I don't suffer as much from the ups and downs of water weight. When I eat a carb-heavy day, even within my 1500-1800 calorie target zone, I will likely see a "gain" on the scale the next few days. That's just how my body works.
So while calories in/calories out is not a hard-and-fast rule, it IS a good place to start. Within that, you need to find out what works best for your body. While "calorie" at its most basic is merely a measure of heat energy -- how much is released from a particular food when it is burned -- your body will react differently to protein than it does to fat or to carbohydrates. It is within those macronutrients that most of the balancing act comes into play.
It sounds more complicated than it really is. Just make a target or target range for your calories and see how that works for a few weeks. See how your body responds. Then you can worry about tweaking the proportion of your macronutrients, if you want to. Or you can go straight low-calorie. that works for a lot of people, too.
~~ Synger ~~