The Harris-Benedict equation is the eldest of them all, initially created in 1919 on the basis of A Biometric Study of Human Basal Metabolism. The BMR values were derived using body surface area (as a function of height and weight), age, and sex.
Men –– 88.4 + (13.4 x weight in kg) + (4.8 x height in cm) - (5.67 x age)
Women –– 447.6 + (9.25 x weight in kg) + (3.10 x height in cm) - (4.33 x age)
Sedentary (little or no exercise): kcal = BMR x 1.2
Lightly active (light exercise/sports 1-3 days/week): kcal = BMR x 1.375
Moderately active (moderate exercise/sports 3-5 days/week): kcal = BMR x 1.55
Very active (hard exercise/sports 6-7 days/week): kcal = BMR x 1.725
Extra active (very hard exercise/sports and a physical job): kcal = BMR x 1.9
Despite several revisions (the latest was in 1984), the accuracy of HBE is still the lowest of the three common equations. It can, probably, be attributed to the fact that the test subjects for the study were all healthy, Caucasian, and most likely of normal weight, as the issues of obesity and extra weight were much less prevalent a century ago. For this reason, various studies show that HBE’s predictions for obese individuals can err by as much as 36% and that the formula’s ability to predict energy requirements varies with weight history and ethnicity.
The Mifflin-St.Jeor equation was derived in 1990 to reflect both the growing prevalence of the obese population in the United States and the demands of a new health-conscious society. Just like the Harris-Benedict equation, the formula assesses 24-hour energy expenditure on the basis of the resting energy expenditure (REE), which is said to account for 65-70% of the total EE, but the study, on which it was based, included both normal weight and obese subjects, and the resulting equation is considered to be more accurate than its predecessor. The study’s authors saw a clear correlation between lean body mass (LBM) and REE, but they deemed measurement of LBM in the out-patient setting generally impractical and concentrated on creating a formula that would be both more accurate than HBE and easy to use.
Women: (10 x weight in kg) + (6.25 x height in cm) - (4.92 x age in years) - 161
Men: (10 x weight in kg) + (6.25 x height in cm) - (4.92 x age in years) + 5
Research has shown that this formula is more likely than others to estimate the resting metabolic rate (RMR) to within 10% of that measured.
There is also the Cunningham equation, which was presented in 1980 and was derived using the same test subjects as the original Harris-Benedict formula. Factors such as sex, age, height, body mass, and estimated LBM were used for analysis, and, according to Cunningham, lean body mass was found to be the single predictor of BMR.
BMR(cal/ day) = 500 + 22 (LBM)
It is considered to be one of the most accurate methods, used especially by athletes, but it is difficult to calculate right as it requires a person to know their body fat percentage and muscle mass. And while the new smart scales promise to provide you with such information, their numbers should be taken with a grain of salt.
That said, the most important thing to remember about any BMR-calculating equation is that whatever formula you choose, it should be viewed only as a very rough estimate of your metabolism, not a definitive measurement. There are a variety of factors that all influence your personal BMR, including genetics (people with a specific FTO gene can burn up to 160 kcal less than other people), resting and sleeping patterns (people with sleep deprivation burn 5-10% less calories), weight loss and gain history, individual eating habits, and the list of medications that you take. Your BMR changes with age and body composition, and as every building block of a healthy lifestyle is a fluid measure that needs to be constantly updated.