These days, you don’t need to calculate your calorie intake for weight loss, because there are plenty of online calculators to do it for you. However, if you don’t have access to the internet, or if you enjoy playing around with numbers, or even if you just like knowing how these things work, it’s possible to work out your calorie maintenance level. This is the number of calories your body requires each day to maintain your current weight without either gaining or losing.
The accepted formula for calculating the calorie maintenance level is the Harris-Benedict Principle. This was outlined in work published by two scientists, J. Arthur Harris and Francis G. Benedict, in 1918. Based on gender, age, height and weight, the Harris-Benedict Equation is used to calculate the Basal Metabolic Rate (BMR). This is the number of calories your body needs to keep you alive and functioning.
Of course, even the most dedicated couch potatoes have some level of activity, so the Harris-Benedict Principle is applied to arrive at the amount of calories your body needs to function at your particular activity level.
To calculate BMR, the equation is:
Women: BMR = 655 + (4.35 x weight in pounds) + (4.7 x height in inches) – (4.7 x age in years)
Men: BMR = 66 + (6.23 x weight in pounds) + (12.7 x height in inches) – (6.76 x age in years)
Let’s apply this equation to a living example – me. I’m a female, weighing 159lbs, 62 inches tall, and 57 years old. Therefore my BMR is:
655 + 691.65 + 291.4 – 267.9 = BMR 1369.75.
To apply the Harris-Benedict Principle, the BMR is multiplied by an increasing factor to arrive at the calories you need to maintain your body at its current activity level. These are the activity levels, with their multipliers. Note they are the same for both men and women.
Sedentary – little or no exercise: calories = BMR x 1.2
Light activity – exercising 1 – 3 days a week: calories = BMR x 1.375
Moderate activity – exercising 3 – 5 days a week: calories = BMR x 1.55
Very active – exercising 5 – 7 days a week: calories = BMR x 1.725
Extremely active – exercising twice a day: calories = BMR x 1.9
As I exercise at least 3 times a week, my activity level is moderate, so my daily calorie requirement is:
1369.75 x 1.55 = 2123.1125. For ease of reckoning, let’s round that calorie requirement down to 2100.
It’s a matter of record that, in order to lose one pound in weight, we need to consume 3500 calories less than our body needs. Most nutrition experts advocate that you aim for a steady weight loss of 1 – 2lbs per week, so if I trimmed my daily calorie intake by 500 a day, I could eat 1600 calories a day, and still expect to lose at least 1lb a week. This weight loss could vary depending on activity levels in specific weeks.
Although the Harris-Benedict Principle is not an exact method of calculation, it’s a pretty good starting point. One slight flaw in the calculation is that it doesn’t take body composition into account. A body with considerable muscle mass needs more calories to maintain itself, while a body of the same weight but carrying a lot of fat needs fewer calories. However, in most cases, the difference should not be significant enough to compromise weight loss.