The first step in designing a personal nutrition plan for yourself is to calculate how many calories you burn in a day; your total daily energy expenditure (TDEE). TDEE is the total number of calories that your body expends in 24 hours, including all activities. TDEE is also known as your "maintenance level". Knowing your maintenance level will give you a starting reference point from which to begin your diet. According to exercise physiologists William McArdle and Frank Katch, the average maintenance level for women in the United States is 2000-2100 calories per day and the average for men is 2700-2900 per day.

These are only averages; caloric expenditure can vary widely and is much higher for athletes or extremely active individuals. Some triathletes and ultra-endurance athletes may require as many as 6000 calories per day or more just to maintain their weight! Calorie requirements may also vary among otherwise identical individuals due to differences in inherited metabolic rates.

Methods of determining caloric needs
There are many different formulas you can use to determine your caloric maintenance level by taking into account the factors of age, sex, height, weight, lean body mass, and activity level. Any formula that takes into account your lean body mass (LBM) will give you the most accurate determination of your energy expenditure, but even without LBM you can still get a reasonably close estimate.

The "quick" method based on total bodyweight
A fast and easy method to determine calorie needs is to use total current body weight times a multiplier.

Fat loss = 12 - 13 calories per lb. of bodyweight
Maintenance (TDEE) = 15 - 16 calories per lb. of bodyweight
Weight gain = 18 - 19 calories per lb. of bodyweight

This is a very easy way to estimate caloric needs, but there are obvious drawbacks to this method because it doesn't take into account activity levels or body composition. Extremely active individuals may require far more calories than this formula indicates. In addition, the more lean body mass one has, the higher the TDEE will be. Because body fatness is not accounted for, this formula may greatly overestimate the caloric needs if someone is extremely overfat. For example, a lightly active 50 year old woman who weighs 235 lbs. and has 34% body fat will not lose weight on 3000 calories per day (255 X 13 as per the "quick" formula for fat loss).

Equations based on BMR
A much more accurate method for calculating TDEE is to determine basal metabolic rate (BMR) using multiple factors, including height, weight, age and sex, then multiply the BMR by an activity factor to determine TDEE. BMR is the total number of calories your body requires for normal bodily functions (excluding activity factors). This includes keeping your heart beating, inhaling and exhaling air, digesting food, making new blood cells, maintaining your body temperature and every other metabolic process in your body. In other words, your BMR is all the energy used for the basic processes of life itself. BMR usually accounts for about two-thirds of total daily energy expenditure. BMR may vary dramatically from person to person depending on genetic factors. If you know someone who claims they can eat anything they want and never gain an ounce of fat, they have inherited a naturally high BMR. BMR is at it's lowest when you are sleeping undisturbed and you are not digesting anything. It is very important to note that the higher your lean body mass is, the higher your BMR will be. This is very significant if you want to lose body fat because it means that the more muscle you have, the more calories you will burn. Muscle is metabolically active tissue, and it requires a great deal of energy just to sustain it. It is obvious then that one way to increase your BMR is to engage in weight training in order to increase and/or maintain lean body mass. In this manner it could be said that weight training helps you lose body fat, albeit indirectly.

The Harris-Benedict formula (BMR based on total body weight)
The Harris Benedict equation is a calorie formula using the factors of height, weight, age, and sex to determine basal metabolic rate (BMR). This makes it more accurate than determining calorie needs based on total bodyweight alone. The only variable it does not take into consideration is lean body mass. Therefore, this equation will be very accurate in all but the extremely muscular (will underestimate caloric needs) and the extremely overfat (will overestimate caloric needs).

Men: BMR = 66 + (13.7 X wt in kg) + (5 X ht in cm) - (6.8 X age in years)

Women: BMR = 655 + (9.6 X wt in kg) + (1.8 X ht in cm) - (4.7 X age in years)

Note: 1 inch = 2.54 cm.
1 kilogram = 2.2 lbs.

Example:
You are female
You are 30 yrs old
You are 5' 6 " tall (167.6 cm)
You weigh 120 lbs. (54.5 kilos)
Your BMR = 655 + 523 + 302 - 141 = 1339 calories/day

Now that you know your BMR, you can calculate TDEE by multiplying your BMR by your activity multiplier from the chart below:

Activity Multiplier
Sedentary = BMR X 1.2 (little or no exercise, desk job)
Lightly active = BMR X 1.375 (light exercise/sports 1-3 days/wk)
Mod. active = BMR X 1.55 (moderate exercise/sports 3-5 days/wk)
Very active = BMR X 1.725 (hard exercise/sports 6-7 days/wk)
Extr. active = BMR X 1.9 (hard daily exercise/sports & physical job or 2X day training, i.e marathon, contest etc.)

Example:
Your BMR is 1339 calories per day
Your activity level is moderately active (work out 3-4 times per week)
Your activity factor is 1.55
Your TDEE = 1.55 X 1339 = 2075 calories/day

Katch-McArdle formula (BMR based on lean body weight) If you have had your body composition tested and you know your lean body mass, then you can get the most accurate BMR estimate of all. This formula from Katch & McArdle takes into account lean mass and therefore is more accurate than a formula based on total body weight. The Harris Benedict equation has separate formulas for men and women because men generally have a higher LBM and this is factored into the men's formula. Since the Katch-McArdle formula accounts for LBM, this single formula applies equally to both men and women.

BMR (men and women) = 370 + (21.6 X lean mass in kg)

Example:
You are female
You weigh 120 lbs. (54.5 kilos)
Your body fat percentage is 20% (24 lbs. fat, 96 lbs. lean)
Your lean mass is 96 lbs. (43.6 kilos)
Your BMR = 370 + (21.6 X 43.6) = 1312 calories

To determine TDEE from BMR, you simply multiply BMR by the activity multiplier:

Example:
Your BMR is 1312
Your activity level is moderately active (work out 3-4 times per week)
Your activity factor is 1.55
Your TDEE = 1.55 X 1312 = 2033 calories

As you can see, the difference in the TDEE as determined by both formulas is statistically insignificant (2075 calories vs. 2033 calories) because the person we used as an example is average in body size and body composition. The primary benefit of factoring lean body mass into the equation is increased accuracy when your body composition leans to either end of the spectrum (very muscular or very obese).

Adjust your caloric intake according to your goal
Once you know your TDEE (maintenance level), the next step is to adjust your calories according to your primary goal. The mathematics of calorie balance are simple: To keep your weight at its current level, you should remain at your daily caloric maintenance level. To lose weight, you need to create a calorie deficit by reducing your calories slightly below your maintenance level (or keeping your calories the same and increasing your activity above your current level). To gain weight you need to increase your calories above your maintenance level. The only difference between weight gain programs and weight loss programs is the total number of calories required.

Negative calorie balance is essential to lose body fat
Calories not only count, they are the bottom line when it comes to fat loss. If you are eating more calories than you expend, you simply will not lose fat, no matter what type of foods or food combinations you eat. Some foods do get stored as fat more easily than others, but always bear in mind that too much of anything, even "healthy food," will get stored as fat. You cannot override the laws of thermodynamics and energy balance. You must be in a calorie deficit to burn fat. This will force your body to use stored body fat to make up for the energy deficit. There are 3500 calories in a pound of stored body fat. If you create a 3500-calorie deficit in a week through diet, exercise or a combination of both, you will lose one pound. If you create a 7000 calories deficit in a week you will lose two pounds. The calorie deficit can be created through diet, exercise or preferably, with a combination of both. Because we already factored in the exercise deficit by using an activity multiplier, the deficit we are concerned with here is the dietary deficit.

Calorie deficit thresholds: How low is too low?
It is well known that cutting calories too much slows down the metabolic rate, decreases thyroid output and causes loss of lean mass, so the question is how much of a deficit do you need? There definitely seems to be a specific cutoff or threshold where further reductions in calories will have detrimental effects. The most common guideline for calorie deficits for fat loss is to reduce your calories by at least 500, but not more than 1000 below your maintenance level. For some, especially lighter people, 1000 calories may be too much of a deficit. The American College of Sports Medicine (ACSM) recommends that calorie levels never drop below 1200 calories per day for women or 1800 per day for men. Even these calorie levels are extremely low. A more individualized way to determine the safe calorie deficit would be to account for one's bodyweight or TDEE. Reducing calories by 15-20% below TDEE is a good place to start. A larger deficit may be necessary in some cases, but the best approach would be to keep the calorie deficit through diet small while increasing activity level.

Example 1: Your weight is 120 lbs.
Your TDEE is 2033 calories
Your calorie deficit to lose weight is 500 calories
Your optimal caloric intake for weight loss is 2033 - 500 = 1533 calories

Example 2:
Your calorie deficit to lose weight is 20% of TDEE (.20% X 2033 = 406 calories)
Your optimal caloric intake for weight loss = 1627 calories

Positive calorie balance is essential to gain lean bodyweight
If you want to gain lean bodyweight and become more muscular, you must consume more calories than you burn up in a day. Provided that you are participating in a weight-training program of a sufficient intensity, frequency and volume, the caloric surplus will be used to create new muscle tissue. Once you've determined your TDEE, the next step is to increase your calories high enough above your TDEE that you can gain weight. It is a basic law of energy balance that you must be on a positive calorie balance diet to gain muscular bodyweight. A general guideline for a starting point for gaining weight is to add approximately 300-500 calories per day onto your TDEE. An alternate method is to add an additional 15 - 20% onto your TDEE.

Example:
Your weight is 120 lbs.
Your TDEE is 2033 calories
Your additional calorie requirement for weight gain is + 15 - 20% = 305 - 406 calories
Your optimal caloric intake for weight gain is 2033 + 305 - 406 = 2338 - 2439 calories

Adjust your caloric intake gradually
It is not advisable to make any drastic changes to your diet all at once. After calculating your own total daily energy expenditure and adjusting it according to your goal, if the amount is substantially higher or lower than your current intake, then you may need to adjust your calories gradually. For example, if your determine that your optimal caloric intake is 1900 calories per day, but you have only been eating 900 calories per day, your metabolism may be sluggish. An immediate jump to 1900 calories per day might actually cause a fat gain because your body has adapted to a lower caloric intake and the sudden jump up would create a surplus. The best approach would be to gradually increase your calories from 900 to 1900 over a period of a few weeks to allow your metabolism to speed up and acclimatize.

Measure your results and adjust calories accordingly
These calculations for finding your correct caloric intake are quite simplistic and are just estimates to give you a starting point. You will have to monitor your progress closely to make sure that this is the proper level for you. You will know if you’re at the correct level of calories by keeping track of your caloric intake, your bodyweight, and your body fat percentage. You need to observe your bodyweight and body fat percentage to see how you respond. If you don't see the results you expect, then you can adjust your caloric intake and exercise levels accordingly. The bottom line is that it’s not effective to reduce calories to very low levels in order to lose fat. In fact, the more calories you consume the better, as long as a deficit is created through diet and exercise. The best approach is to reduce calories only slightly and raise your daily calorie expenditure by increasing your frequency, duration and or intensity of exercise.