If you have ever used an online TDEE or calorie calculator, the math behind your BMR result almost certainly came from the Mifflin-St Jeor equation. It is the most widely validated predictive formula for resting metabolic rate in non-athlete adults, and it replaced the older Harris-Benedict equation as the clinical reference standard in most research contexts after comparative analyses found it to be more accurate across broader population ranges.
Understanding how the equation works gives you a clearer picture of what your BMR estimate actually represents, where its limitations are, and why the inputs matter as much as they do.
The Formula
The Mifflin-St Jeor equation calculates BMR in kilocalories per day using four variables: weight in kilograms, height in centimeters, age in years, and biological sex.
For males:
BMR = (10 * weight_kg) + (6.25 * height_cm) - (5 * age_years) + 5
For females:
BMR = (10 * weight_kg) + (6.25 * height_cm) - (5 * age_years) - 161
The only difference between the two formulas is the constant at the end: +5 for males and -161 for females. This offset reflects average differences in body composition, specifically lean mass relative to total body weight, between the two groups at equivalent measurements.
If you want to verify a calculator result by hand, the arithmetic is straightforward. A 35-year-old male who weighs 80 kg and stands 175 cm tall would calculate BMR like this:
(10 * 80) + (6.25 * 175) - (5 * 35) + 5
= 800 + 1093.75 - 175 + 5
= 1723.75
Rounded to approximately 1,724 kilocalories per day.
What Each Coefficient Represents
Each variable in the equation carries a coefficient that reflects its relative contribution to resting metabolic rate.
Weight (x 10): Body mass is the strongest single predictor of BMR. More tissue requires more energy to maintain cellular function at rest. Each additional kilogram adds roughly 10 calories per day to your resting burn, making weight the dominant variable in the equation.
Height (x 6.25): Taller individuals have more surface area and a larger volume of metabolically active tissue at any given weight. Height accounts for this body volume difference independently of weight, which is why two people with the same weight but different heights produce different BMR estimates.
Age (x -5): Resting metabolic rate declines with age, even when body weight holds steady. The -5 coefficient means that each additional year reduces estimated BMR by approximately 5 calories per day. By the time someone is 60, their BMR estimate is about 175 calories lower than it would have been at 25 with identical weight and height inputs.
Sex constant (+5 or -161): This is a fixed offset accounting for average differences in body composition between males and females at the population level. It does not mean any particular woman has a BMR exactly 166 calories lower than an equivalent man; it reflects an average population-level difference driven primarily by differences in muscle mass distribution.
Why Weight Is the Dominant Variable
Weight accounts for more of the variation in BMR between individuals than any other input in the formula. This is why body weight change has such a significant effect on calorie requirements. When someone loses 10 kg, their BMR drops by approximately 100 calories per day, not because their metabolism is impaired but because there is less tissue to sustain.
This is also why BMR-based calorie targets need to be recalculated as weight changes rather than treated as fixed numbers. A calorie target calibrated at 90 kg will be meaningfully off by the time someone reaches 78 kg, because the underlying equation inputs have changed substantially.
Where the Formula Has Limitations
The Mifflin-St Jeor equation predicts BMR from external measurements without directly assessing body composition. It assumes a typical ratio of lean mass to fat mass for a given weight and height combination. Two people with identical inputs can have meaningfully different actual BMRs if one has substantially higher lean mass than average for their measurements.
Athletes with high muscle mass tend to have higher actual BMRs than the equation predicts, because muscle tissue is metabolically more active than fat tissue at rest. Individuals with higher body fat percentages at a given weight may have lower actual BMRs than the equation suggests.
Clinical studies have found that the Mifflin-St Jeor equation predicts measured BMR within about 10% for the majority of the non-athlete adult population. That level of accuracy is sufficient for setting calorie targets, diagnosing stalls, and using TDEE as a practical planning tool. For elite athletes or clinical metabolic assessment, direct measurement using indirect calorimetry provides more individual precision.
The National Institute of Diabetes and Digestive and Kidney Diseases publishes accessible resources on how energy expenditure is measured and estimated. The Harvard T.H. Chan School of Public Health maintains research-backed information on energy balance and nutritional metabolism. The Centers for Disease Control and Prevention provides additional context on physical activity and metabolic health that frames where BMR estimates fit in the broader picture.
From BMR to TDEE
BMR is an intermediate calculation. To get the number you actually use for daily calorie targets, you multiply BMR by an activity factor. The standard categories range from sedentary (BMR x 1.2) to extremely active (BMR x 1.9).
For the 35-year-old male in the example above, a sedentary lifestyle would produce a TDEE of approximately 2,069 calories. At a moderately active level (BMR x 1.55), that rises to approximately 2,671 calories. The activity multiplier has a larger practical effect on the final TDEE than small errors in the BMR estimate itself, which is why selecting the right activity level matters more than refining the BMR calculation.
You can run the full calculation, including BMR, TDEE, and goal-adjusted macro targets, using this free calculator at EvvyTools. The TDEE Calculator is available at evvytools.com/tools/health-fitness/tdee-calculator/ and handles the Mifflin-St Jeor equation while returning results for your selected activity level.
For a practical guide on applying TDEE results to fat loss, maintenance, or muscle building goals, the article at evvytools.com/blog/how-to-calculate-tdee-daily-calorie-target/ covers the activity multiplier selection, goal-adjusted calorie targets, and macro distribution in detail.
Photo by PublicDomainPictures on Pixabay
A Note on Input Accuracy
Because weight carries the largest coefficient (x 10), inaccurate weight inputs have the most direct effect on BMR. Most people can accurately self-report their weight within 0.5-1 kg, which translates to a 5-10 calorie error in BMR. More significant in practice is the choice of activity multiplier applied after BMR is calculated, which can shift TDEE by several hundred calories based on one category difference.
Height and age inputs are typically stable and accurate. The formula itself performs consistently when inputs are honest. The most consequential variable to get right is not a coefficient in the equation but the activity level judgment applied to convert BMR into TDEE.
Comparing Mifflin-St Jeor to Other Formulas
The other commonly encountered BMR formula is Harris-Benedict, published in 1919 and updated in 1984. The revised Harris-Benedict formula is slightly more complex than Mifflin-St Jeor and tends to overestimate BMR in obese individuals. Head-to-head comparisons consistently find Mifflin-St Jeor more accurate across a wider range of body compositions, which is why clinical dietetics and health research have largely converged on it as the standard for non-athlete populations.
The Katch-McArdle formula is a third option. It requires body fat percentage as an input, which makes it more accurate for people who know their composition precisely (typically those with DEXA scan or hydrostatic weighing data). Without accurate body fat data, Mifflin-St Jeor is generally the more reliable choice.
For most people using an online calculator to estimate TDEE, the Mifflin-St Jeor equation gives a starting point that is accurate enough to be useful and only needs periodic recalculation as weight and activity level change.
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