For weight to remain stable, energy intake must match energy output. This energy is derived from the foods that we eat. The value for food energy is expressed in food calories (sometimes abbreviated as “kcal”) or kilojoules (kJ). One kcal is equal to 4.1868 kJ. The average energy intake for American men is about 2800 kcal/d and 1800 kcal/d for American women. However, this estimate can vary depending on body size and activity level. [1]

Now, wouldn’t it be great if you can determine your own daily caloric requirement? With just a simple play of numbers, you can actually compute how much food calories your body needs everyday. In doing so, you will have an idea if you have eaten more or less than what you actually needed.

Try these 2 simple steps for calculating your own daily caloric requirement.

#1 Determine your Resting Energy Expenditure (REE) or Basal Metabolic Rate. This is the number of calories you would use if you spent the whole day lying in bed.
For males:     REE = 900 + 10(w)
For females:  REE = 700 + 10(w)
where w is weight in kilograms

#2 The calculated REE is adjusted for physical activity level. Multiply REE with these factors:
1.2 ? for sedentary (no exercise, sit at a desk most of the day)
1.4 ? for moderately active(exercise 3 or more days a week for 30 minutes or more)
1.8 ? for very active individual (exercise 5 or more days a week for 30 minutes or
more)

The answer you get provides an estimate of how many calories you need to maintain your current weight.

Here is an example: Coede is a 58 year-old businessman, 5’6” tall, and weighs 54 kilograms. He plays tennis three times a week.

REE = 900 + 10 (w)
= 900 + 10 (54)
= 1440
Then, multiply 1440 by 1.4, since Coede’s physical activity level is categorized as moderately active. The answer is 2,016.

So, to maintain Coede’s present weight, he should eat a total of about 2,016 calories a day.

The question now is: How do you know the number of calories present in your food that you eat everyday? The best way to keep track of how much calories are in the food that you eat is to look at the Nutrition Fact panel printed at the back of every food product packages. Add all the calories that you take in for the entire day and see if it is as close as possible to your daily caloric requirement. Remember that only carbohydrates, fats, proteins, polyols, organic acids, and ethanol contain calories. One gram of fats contains 9 kcal/g, polyols have 7 kcal/g, carbohydrates have 4 kcal/g, proteins have 4 kcal/g, and organic acids have fewer than 4 kcal/g. Everything else in food is non-caloric.

Reference:
Kasper, Dennis, et.al. (2005). Nutritional Requirements and Dietary Assessment.
Harrison’s Principles of Internal Medicine 16th edition