A fast and easy method to determine calorie needs is to use total current body weight times times calorie requirements.

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 easy way to estimate caloric needs, but it doesn’t take into account activity levels or body composition. Extremely active individuals may require a lot more calories than this formula indicates. The more lean body mass a person has, the higher the TDEE (maintenance level) 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 pounds and has 34% body fat will not lose weight on 3000 calories per day (235 X 13 as per the “quick” formula for fat loss).

How to Calculate Basal Metabolic Rate (BMR) 

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 uses 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 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.

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). Including keeping your heart beating, Breathing, digestion, maintaining body temperature and every other metabolic process in your body. BMR usually accounts for about two-thirds of total daily energy expenditure. BMR can 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.  Your BMR is at it’s lowest when you are sleeping as long as you did not eat really late before bed. The higher your lean body mass is, the higher your BMR will be. This is really important if you want to lose body fat because it means that the more muscle you have, the more calories you burn. Muscle is metabolically active tissue, and needs lots of energy just to sustain it. One way to increase your BMR is to engage in weight training in order to increase or maintain lean body mass.

 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 means its more accurate than using bodyweight alone. However because it does not take into account lean body mass which means it will not be good for people who are extremely muscular because it will under estimate calorie needs and the extremely overfat  it 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 years 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

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. To lose weight, you need to create a calorie deficit by reducing your calories slightly below your maintenance level or you can increase activity. To gain weight you need to increase your calories above your maintenance level.

Negative calorie balance is essential to lose body fat.

If you are eating more calories than you use in a day, you will not lose fat, no matter what foods you eat. There are foods that can be stored as fat easily but remember eating more than you use will end in weight gain even if its healthy foods. 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

How low is too low?

Cutting calories too much slows down the metabolic rate, decreases thyroid output and causes loss of lean mass, so how much of a deficit do you need? A 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. It is recommended that women never eat under 1200 calories per day or 1800 per day for men, and 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. The best way 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

 Go Slow adjusting caloric intake.

Don’t be tempted to make dramatic cuts in calories. Calculate 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. This Means if your calorie intake should be 2500 but you only eat 1100 calories jumping to 2500 calories may cause you to gain weight because your metabolism has been sluggish due to the low calories you have been eating. The best approach would be to gradually increase your calories from 1100 to 2500 over a period of a few weeks to allow your metabolism to speed up and get used to the new changes.

Measure and adjust calories accordingly

Remember to monitor your progress closely to make sure that you are eating optimanly for your goals. You need to observe your bodyweight and body fat percentage to see how you are responding. If you don’t see the results you expect, then you can adjust your caloric intake and exercise levels accordingly. Remember reducing calories to lose fat to very low levels is detrimental to fat loss. In fact, the more calories you consume the better, as long as a deficit is created through proper nutrition and exercise. The best way is to reduce calories slightly and increase activity to increase your daily expenditure.