The Harris-Benedict formula is used to determine your basal metabolic rate (BMR), which is also called your resting energy expenditure (EER). Your basal metabolic rate is determined by your gender, age and body size, and calculating this number tells you how about how many calories you burn just being alive and awake.
Of course, since you get out of bed and move around every day, you need to adjust this number. You can determine your active metabolic rate (AMR) by multiplying your BMR by a number representing on your activity levels. This number ranges from 1.2 for being sedentary, up to 1.9 for being extra active. Here's how it works:
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) - (6.8 x age in years)
Calculate your active metabolic rate by starting with your basal metabolic rate and adjusting it by estimating your current level of activity. If you are:
- Sedentary (little or no exercise) - your AMR = BMR x 1.2
- Lightly active (light exercise/work 1-3 days per week) - your AMR = BMR x 1.375
- Moderately active (moderate exercise/work 3-5 days per week) - your AMR = BMR x 1.55
- Very active (hard exercise/work 6-7 days a week) - your AMR = BMR x 1.725
- Extra active (very hard exercise/work 6-7 days a week) - your AMR = BMR x 1.9
Your AMR should represent the number of calories you need to consume each day to stay at your current weight. If you want to lose weight, you need to increase your level of physical activity or decrease your caloric intake by eating less.
Accuracy Problems With the Harris - Benedict Formula
Unfortunately, the Harris - Benedict formula isn't perfect. According to the Academy of Nutrition and Dietetics, research studies have indicated the formula is about 90 percent accurate around 60 percent of the time.
That means it could be way off about 40 percent of the time, which is rather disheartening. Even worse, when the formula was wrong, it overestimated the calorie needs of the research subjects, so they were burning fewer calories than they thought.
The formula's problems with accuracy may be due to physical or genetic factors, and there's a good chance that many people overestimate how physically active they are.
So why use the formula if it's not always accurate?
Because it's a good place to start. You can calculate your daily caloric need, and if you don't lose or gain weight, you can adjust your daily calorie goal, but please don't consume fewer than 1,200 calories per day without medical supervision.
Academy of Nutrition and Dietetics Evidence Analysis Library. "In non-obese individuals, what is the prediction accuracy and maximum overestimation and underestimation errors compared to measured resting metabolic rate when using the Harris-Benedict formula?" Accessed August 7, 2012. http://www.adaevidencelibrary.com
Crystal C. Douglas, Jeannine C. Lawrence, Nikki C. Bush, Robert A. Oster, Barbara A. Gower, Betty E. Darnell. "Ability of the Harris Benedict formula to predict energy requirements differs with weight history and ethnicity." Nutr Res. 2007 April; 27(4): 194-199. doi: 10.1016/j.nutres.2007.01.016.