Basic Definitions and Dietary Information.
This article provides an outline of how to determine how much food your body needs each day, and from what sources. The methods outlined in this article are accurate but not precise. They will provide you with guidelines from which you can begin experimentation, but not definite answers. In truth there is only one way to determine precisely your food needs and that is a total metabolic analysis including 24 hour monitoring of heart rate, oxygen usage, body temperature, blood glucose levels, hormonal and blood micronutrient analysis and so on. In short, you cannot precisely determine your needs outside of a laboratory environment, but you can get an accurate estimate. In order to make sense of the information contained within you will need to know a few simple definitions as follows:
Calorie (Kcal)  A unit of energy. Almost all substances you consume provide energy to your body, and the calorie is simply the standard measurement we use for the energy provided in a food.
Macronutrient  “Macro” equates to large, so macronutrient simply means “large nutrient”. There are three macronutrients: Protein, carbohydrates and fats.
Micronutrient  “Micro” equates to small, so micronutrient simply means “small nutrient”. There are hundreds of micronutrients and they include all Vitamins, Minerals, enzymes and cofactors.
Metabolism  The sum total of all the actions and reactions that occur in your body.
Basal Metabolic Rate  The amount of energy you utilize while completely at rest but not asleep.
Metabolic Rate  The amount of energy you use in total i.e. including all your activity.
Determining Your Daily Calorie Needs.
Here is the method I use to calculate calorie needs when I don't have time to do an hourbyhour analysis. It is slightly rough, but never more than 100 or so Kcals away from the answer you get by determining basal metabolic rate then performing hourly work output measurements. Use it as you progress towards your goals to enable you to reassess your calorie needs as you lose fat and build muscle.
Step 1
For Men: 
1 x body weight (kg*) x 24 = _________ 
For Women: 
.9 x body weight (kg*) x 24 = _________ 
* to convert pounds to kg, divide weight in pounds by 2.2

Step 2
In this step you must multiply the result from step 1, by the "multiplier" in the right column that corresponds to your body fat percentage. This step takes into account that pound for pound, leaner people generally have a higher Basal Metabolic Rate (BMR). BMR is the amount of energy you use simply to sustain your body when completely inactive.
Table 1
Percent Body Fat 
Multiplier 
Men 10 to 14% ,Women 14 to 18% 
1.0 
Men 14 to 20% ,Women 18 to 28% 
.95 
Men 20 to 28% ,Women 28 to 38% 
.90 
Men over 28% ,Women over 38% 
.85 
Example:
Sex: 
Male 
Weight: 
200 pounds (90.9 kg) 
Body Fat: 
15% 
*To find weight in kg, divide the body weight in pounds by 2.2
200 pounds divided by 2.2 = 90.9 kg

Step 1
1 x 90.9 x 24 = 2182
Step 2
2182 x .95 (multiplier from table 1 corresponding to 15% body fat) = 2073
BMR = 2073 calories / day
Step 3
Now that you know the BMR, the final step in determining daily caloric expenditure is to factor in your daily activity. Obviously, activity means you are going to burn more calories than the BMR. Find where you fall in the activity categories below and multiply your daily BMR by the number on the left of your range to find your daily caloric expenditure.
Average Daily Activity Levels
The Average Couch Potato range:
1.30 (130%) = Very Light: Sitting, studying, talking, little walking or other activities through out the day
1.55 (155%) = Light: Typing, teaching, lab/shop work, some walking throughout the day
The Average Fitness Buff Range:
1.55 (155%) = Light: Typing, teaching, lab/shop work, some walking throughout the day
1.65 (165%) = Moderate: Walking, jogging, gardening type job with activities such as cycling, tennis, dancing, skiing or weight training 12 hours per day

The Average Athlete or Hard Daily Training Range:
1.80 (180%) = Heavy: Heavy manual labor such as digging, tree felling, climbing, with activities such as hockey, soccer or body building 24 hours per day
2.00 (200%) = Very Heavy: A combination of moderate and heavy activity 8 or more hours per day, plus 24 hours of intense training per day


Example:
Using the same 200 pound male with a daily BMR of 2073 calories per day, we determine that he is mostly sedentary during the day with the exception of some jogging and about 40 minutes of weight training. This would put him in the activity category of 1.65, or 165% of his BMR.
Total daily caloric expenditure = 2073 calories x 1.65 = 3420 calories
Our 200pound male at 15% body fat and a BMR of 2073 calories per day uses a total of 3420 calories on an average day.
Now that you know your approximate calorie needs you must find out how many calories are going to come from each of the three macronutrients: Protein, Fats, and Carbohydrates.
Determining Proper Ratios of Protein, Fat and Carbohydrates
Now that we know how many total calories need to be consumed each day, where do the calories come from? In the diet there are three worthwhile sources of calories: fat, protein and carbohydrates. There are many different ratios, and methods of determining those ratios, in use today and needs will vary for every individual depending upon body weight, activity level, genetic factors and even age. The first thing we need to know is how many calories there are in each of the macronutrients?
Calories per gram of the three major nutrients: 
Protein 
4 calories per gram 
Carbohydrates 
4 calories per gram 
Fat 
9 calories per gram 
As you can see from the table fat is by far the most calories dense of the nutrients, providing over twice the calories per gram of either protein or carbohydrates. This is why those seeking to lose weight or almost always told to cut down on fat intake. It is the easiest way to reduce calorie intake.
In order to determine how much of each macronutrient you need you will have to do some simple math. The equations for each macronutrient are listed below along with brief descriptions of their origin and the reasoning behind their use. We begin with protein needs, then carbohydrates and fats together. The way I see it is that carbohydrates and fats are both simply sources of energy in the body. After you have found your protein needs it is simply a matter of finding what ratio of carbohydrates to fats work best for your body. This varies from person to person and finding what works for your own body is primarily a matter of trial and error.
Determining Protein Needs.
As outlined above needs vary according to many factors, primarily lean body weight and activity level. The table below is an excellent outline for determining protein requirements developed by Fred Hatfield, a respected American sports scientist with many years of “in the trenches” real world experience including a 1014lb Squat in Powerlifting competition. This man knows his stuff both in the lab and in the “real world” and I endorse his advice wholeheartedly. In order to make use of the table you will need to know your lean body weight. To find this, simply have your body fat percentage measured, and then do the necessary math. Say for example you weigh 10 stone (140lbs) with a body fat percentage of 15%. If 15% of your weight is fat then logically the remaining 85% is lean tissue, so .85 x 140lbs = 119lbs, which is your lean body weight. Do the calculation for your own body, and see from the table what your protein needs really are each day.
Hatfield's Estimate For Determining Daily Protein Requirements.
Formula: Lean Body Weight x Need Factor = Daily Protein Requirement in Grams.
Need Factors:
0.5  Sedentary, no sports or training.
0.6  Jogger or light fitness training.
0.7  Sports participation or moderate training 3 times per week.
0.8  Moderate daily weight training or aerobic training.
0.9  Heavy weight training daily.
1.0  Heavy weight training daily plus sports training, or “2aday” training.
Daily Protein Requirement = ________g.
As always, the figure you obtain from this calculation is a starting point. Use it as a basis for determining your protein needs and then experiment from there with greater amounts if you feel you may benefit. A percentage of people may need more protein than this, but only a few, for instance adolescent athletes, those with extremely demanding jobs e.g. manual labor in addition to training, or those whose goals include the addition of maximal muscle mass e.g. bodybuilders. These groups can use up to 1.5 as a "need factor".
Carbohydrate And Fat Needs.
There are several possible ways to calculate how much carbohydrate you need in your diet. Thankfully, one of the simplest ways also tends to be the most accurate. First of all you will need to determine your total calorie needs for an average day. Once you know your total calorie needs you will have to subtract he calories taken up by protein and fats. The protein value can be deduced by using The Hatfield Estimate For Determining Daily Protein Requirements. Once you know your daily protein needs in grams, and your total calorie needs, use the following equation to determine carbohydrate needs.
Daily protein requirement in grams = ______ x 4 = _________ (number of Kcal Protein)
Daily Calorie need = _______.
(.7 to .9) x Daily Calorie Need = ______  Kcal Protein_______ = _______Kcal Carbs.
Kcal Carbs / 4 = _______Grams of Carbs per day.
In order to determine how many calories are going to come from carbs you will have to decide upon a fixed percentage of calories that are to come from fats. Anything between 10% and 30% of total calories can come from fats without causing undue risks to health. If you perform a lot of aerobic exercise you may wish to keep your fat intake lower in order to maximize the amount of carbs you can take in. If you are particularly sensitive to carbohydrates then you may wish to keep fat intake higher and lower your carb intake accordingly. As stated before it is a matter of trial and error. It is this fixed percentage of fats that gives you the number between .7 and .9 in the last step of the equation. For instance if you decide to obtain 10% of your total calories from fats, then 90 percent (.9) will come from protein plus carbohydrate. If you were to obtain 20% from fats then the remaining 80% (.8) would come from protein plus carbohydrates. So what you are doing is to take the number of calories you will get from carbs plus protein, then subtracting the protein calories (Kcal protein). The remaining figure is the number of calories you will need to obtain every day through carbohydrates. As there are 4 calories per gram of carbohydrate you will have to divide this figure by 4 to find out how many grams of carbs you need each day. All that remains is to work out how many grams of fat make up your daily intake. Simply take the percentage figure you decided upon (10 to 30 % of total calories) and, as fat has 9 Kcal per Gram, divide it by 9. This gives you your daily fat intake in grams. Finding the appropriate balance between carbohydrates and fats is typically a matter of experimentation Based on individual insulin sensitivity. Some people very readily deposit calories from carbohydrates as body fat. Others are far more likely to store calories from fat as body fat. Nobody else can tell you how you respond to each nutrient. You have to deduce it for yourself. A few of the more common ratios of macronutrients are presented below along with their individual advantages and disadvantages. This may provide you with some indication of where to begin with regard to your own personal ratio of carbohydrates to fats.