The Harris-Benedict equation was published in 1919, derived from a study of 136 men and 103 women — a small sample by modern research standards, measured with the metabolic equipment available a century ago. It remained the default formula for estimating Basal Metabolic Rate (BMR) in clinical and fitness settings for over 70 years, largely because no widely validated alternative existed.
The 1990 Revalidation Study
In 1990, researchers Mark Mifflin and Sachiko St Jeor revisited the question with a much larger sample — 498 healthy adults — and compared predicted BMR against measured BMR using indirect calorimetry, the gold-standard method of measuring metabolic rate from oxygen consumption. The Harris-Benedict equation, applied to this modern sample, overestimated actual measured BMR by an average of 5%, and more for individuals with higher body fat percentages.
The overestimation made sense in hindsight: the original 1919 study population likely had different average body composition than a 1990 (or 2026) population, and body composition — specifically lean mass — is the primary driver of metabolic rate, not weight alone. Harris-Benedict used only weight, height, age, and sex, with no way to account for muscle-versus-fat differences between individuals or generations.
The Mifflin-St Jeor Formula
For men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
For women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
The structure looks similar to Harris-Benedict — both use weight, height, age, and a sex-based constant — but the coefficients were recalibrated against the larger, more rigorously measured dataset. Subsequent validation studies through the 2000s and 2010s have consistently found Mifflin-St Jeor to be more accurate than Harris-Benedict across a wider range of body compositions, which is why it has become the standard formula recommended by the Academy of Nutrition and Dietetics.
Neither Formula Accounts for Muscle Mass Directly
Both formulas estimate BMR from weight, not lean body mass specifically. For most people with average body composition, this is a reasonable approximation. For individuals with unusually high muscle mass (bodybuilders, strength athletes) or unusually high body fat percentage, formulas that incorporate measured lean mass — such as the Katch-McArdle formula — tend to be more accurate, but they require a body fat percentage measurement as an input, which introduces its own measurement error.
For most practical purposes, Mifflin-St Jeor remains the most reliable formula without requiring a body composition scan. Use the USECALC TDEE Calculator, which applies Mifflin-St Jeor as its base BMR formula before layering on your activity multiplier, to get your estimated daily calorie needs.