This is in my opinion, the most accurate way to identify your ideal calorie intake. The only downside is that it takes a little more work from you to get it right.
Weigh yourself after your morning bowel evacuation. Note this weight.
Catalog your diet in a nutritional log, such as the one found here, for an entire week.
After a week of following your controlled nutrition plan weigh yourself again and see what the difference is.
If you've gained a lb, then you are approximately 500 calories per day above your daily caloric maintenance level, assuming you ate the same # of calories each day. (½ lb over = 250 cal over, 2 lbs = 1000 cal, etc). If you've maintained your present weight, then you are eating approximately at your maintenance level (calories in = calories out). If you've lost a pound, then you're 500 calories below your maintenance level.
Determine your maintenance level. From there, adjust your calories for your weight gain/loss goal. +500 kcal daily to gain 1-lb weekly, +750 kcal daily to gain 1.5 lbs weekly, +1000 to gain 2-lb weekly (don't do this if you're over 25, you'll get fat), +1500 if you want to gain 3-lb weekly (don't do this if you aren't still growing in height, you will get fat, unless you are a mutant). This is NOT 100% IRONCLAD, but is a pretty easy and cheap way to get the ball rolling.
This works in combination with the above methods too. You can make a quick determination of daily calories with Method 1 or 2 and then use Method 3 to check the efficacy of your menu. I recommend doing this every week or two and reassessing where your calorie needs stand.
How many Proteins, Carbs and Fats should I be getting?Edit
Skinny dudes probably will want higher carb and fat levels, and can shoot for about 25-50-25 for their PRO-CHO-FAT ratios. This means, 25% of calories will come from protein, 50% from carbohydrates and 25% from fat. This is NOT an exact specification! Skinny dudes don't need to follow the "super-high protein" type diets. You simply won't build muscle all that fast. You'll need the carbs and especially the fats to keep your body from catabolizing muscle tissue to use as fuel, just make sure you have a steady supply of nutrients entering your body during the day. NO SKIPPED MEALS!!!!
Chubbies will want lower carb levels and higher protein levels. give 50-30-20 or 50-25-25 a try and see how that works for you. Again, no skipped meals.
Natural mesomorphs (i.e. athletic types, those who are naturally pretty strong and lean) can probably do best (or do real well) on a diet that is somewhere around 40-40-20 of protein-carb fat. To be honest, almost anything will work for these guys, as long as they have their caloric needs met throughout the day.
Almost everyone can do pretty well on a 30-40-30 or a 33-33-33 type diet as well, assuming the carbs are clean (specifically this applies to chubbies).
Are these absolutes? No, of course not; they are starting points. Use them as such. If you know that you don't respond well to those same ratios, then great! Congrats. You already know what to eat, why are you reading this? :p
Let's do the calculations for a skinny 150-lb teenager, using the "Body Type Method".
150lbs x 22kcal/lb = 3300 calories.
25% protein = .25 x 3300 = 825 calories. 825 calories divided by 4 calories/gram ~ 205 grams PRO.
50% CHO = .50 x 3300 = 1650 calories. 1650 / 4 ~ 410-415 grams CHO
25% fat = 825 calories. 825/ 9 ~ 70-75g FAT.
That is the BASELINE. You will almost definitely want to add to this, especially because you have to account for the extra calories you are burning during training. Chances are good skinnies will want to add to the carbs andespecially the fats.
Eating a ton does NOT mean you're absorbing a ton. You have to properly absorb your calories in order for them to be of use. If you are farting and crapping yourself every 10-15 minutes, then you added too many calories too fast. Scale back a bit and work your way back up. Too much too soon can overload your system. A good digestive enzyme can help remedy this.
You also may have a food allergy (wheat gluten and dairy lactose are 2 major culprits here) There are volumes upon volumes written about diet, go read up and learn more for yourself.
It takes approximately 3500 calories above maintenance to add a pound of bodyweight in a week (3500 calories/7 days = 500 calories/day).
It is consistent with typical muscular weight gains to put on .5-1.5 lbs. per week. If you're gaining more than that, and you're not a teenager, chances are you are gaining more than a bit of fat. If you're not a teenager and are gaining more than 2 lbs per week and it's muscle, then you're either a) a genetic freak, b) on steroids, or c) a genetic freak on steroids. Only teenagers (damn them all) have this unique ability.
Eat too few calories, your gains will suffer. Eat too many calories, you'll add fat. As far as gains go it's better to overeat. In the end, it's up to you to determine your sweet spot as far as total calories. Keep a food log if you want to see how many actual calories you're eating, it's VERY difficult to estimate correctly.