BMR (Basal Metabolic Rate) is the number of calories your body burns at complete rest, and since it's not something you can measure without specialized lab equipment, it's almost always estimated using a formula built from weight, height, age and sex. Two formulas dominate: Harris-Benedict, the older and long-standing standard, and Mifflin-St Jeor, developed decades later specifically to correct known weaknesses in the original. They routinely produce different numbers for the same person, and the gap comes directly from how each formula was built.
Where each formula comes from
The original Harris-Benedict equation was published in 1919, based on a study of a relatively small and not particularly diverse group of subjects by modern standards, and was revised in 1984 to adjust its coefficients slightly. Mifflin-St Jeor, published in 1990, was built from a larger and more contemporary sample, and was specifically designed to address the tendency of Harris-Benedict to overestimate BMR for many people, a gap that had grown as average body composition in the population shifted over the 20th century.
The formulas themselves differ in their exact coefficients. Mifflin-St Jeor for men is roughly (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5, and for women the same expression minus 161 instead of plus 5. Harris-Benedict uses different multipliers and a different structure entirely. Because the underlying constants come from different data, the two formulas can diverge by 100-200 calories or more for the same person, particularly at higher body weights.
Why Mifflin-St Jeor is generally preferred today
Multiple validation studies comparing both formulas against measured resting energy expenditure (using indirect calorimetry) have found Mifflin-St Jeor to be more accurate on average for contemporary populations, which is why it's become the default in most clinical and calculator settings, including the BMR Calculator. That said, "more accurate on average" doesn't mean exact for every individual, both formulas are estimates built from population averages, and any single person's actual BMR can differ from either formula's prediction due to factors like genetics, muscle mass, and thyroid function that neither formula accounts for directly.
Which number should you actually use?
For most people, the practical answer is to pick one formula, generally Mifflin-St Jeor given the accuracy research, and use it consistently rather than switching between formulas and being confused by the different outputs. Once you have a BMR estimate, it becomes the base number for calculating TDEE, since TDEE is BMR multiplied by an activity factor, so a more accurate BMR feeds directly into a more accurate total calorie estimate for weight management.