Find Your BMR — The Calories Your Body Burns at Rest

Two equations dominate BMR estimation in nutrition science: Mifflin-St Jeor (1990) and Harris-Benedict (1919, revised 1984). Both take the same four inputs — weight, height, age, and sex — but they produce different results, and understanding why matters if you want accurate calorie targets.

Harris-Benedict was the first widely adopted BMR formula, originally published in 1919 by James Arthur Harris and Francis Gano Benedict. It was revised in 1984 by Roza and Shizgal to correct for measurement errors in the original sample. For decades it was the clinical standard, and it remains embedded in many hospital nutrition software systems simply because it was there first. That institutional inertia is the main reason clinicians still use it — not because it is more accurate.

Mifflin-St Jeor was developed in 1990 by M.D. Mifflin and S.T. St Jeor using a larger, more diverse sample of subjects. The equations were designed specifically to address the known shortcomings of Harris-Benedict and were validated against measured resting energy expenditure using indirect calorimetry.

The practical difference between the two comes down to systematic bias. Harris-Benedict tends to overestimate BMR by 3–7%for normal-weight individuals. That might sound small, but on a 1,800 kcal BMR it translates to 54–126 extra calories per day — enough to stall fat loss or cause unexpected weight gain if you're building a calorie target on top of an inflated baseline.

The most cited evidence comes from a 2005 meta-analysis by Frankenfield et al. published in the Journal of the American Dietetic Association. The study reviewed predictive equation accuracy across multiple populations and found that for non-obese adults, Mifflin-St Jeor produced a mean error of approximately 1%, while Harris-Benedict showed a mean error closer to 5%. The authors concluded that Mifflin-St Jeor is the most accurate predictive equation for resting energy expenditure in this population.

When should you use Harris-Benedict? If you are in a clinical setting where the software uses HB and there is no option to switch, or if you are comparing results to an older study that used HB as its reference equation. For personal nutrition planning, there is no compelling reason to prefer HB over MSJ.

The practical implication is straightforward: use Mifflin-St Jeor as your default. If your calorie target feels wrong — if you are eating at what you calculated as maintenance and still gaining or losing weight — the answer is not to switch formulas. It is to track your actual weight trend for two to four weeks and adjust your target from real data. Equations are a starting point, not a verdict.