Basal Metabolic Rate: Formula, Calculation, and Accuracy

Basal Metabolic Rate Formula: Mifflin-St Jeor

The Mifflin-St Jeor equation estimates resting energy expenditure from weight in kilograms, height in centimeters, and age in years. Published by Mifflin et al., 1990 in the American Journal of Clinical Nutrition, the method provides separate mathematical formulas for men and women.

The only difference between the two equations is the final constant. For men, the formula adds 5. For women, the formula subtracts 161. That creates an exact gap of 166 kcal/day when evaluating a man and a woman of the same weight, height, and age. That gap represents an average across a population. It is not a statement about any single person. This difference is generally attributed to women tending to carry more body fat and less skeletal muscle than men of comparable weight.

Worked Example for Calculating Basal Metabolic Rate

Enter weight in kilograms, height in centimeters, and age in years. Take a 35-year-old man who weighs 80 kg and stands 180 cm tall. Work out each part, then add the male constant:

• Weight calculation: 10 x 80 = 800
• Height calculation: 6.25 x 180 = 1,125
• Age calculation: 5 x 35 = 175
• Male constant: + 5

Together: 800 + 1,125 - 175 + 5 = 1,755 kcal/day. Now use the women's formula with the same inputs. The three baseline parts total 1,750 (800 + 1,125 - 175). Subtracting the female constant of 161 produces 1,589 kcal/day (800 + 1,125 - 175 - 161 = 1,589 kcal/day). The difference between 1,755 kcal/day and 1,589 kcal/day equals the standard 166 kcal/day constant gap.

BMR vs RMR vs Total Daily Calories

Basal metabolic rate and resting metabolic rate differ primarily in the clinical conditions required to record them. BMR represents the minimum energy needed to sustain basic body functions at rest, strictly measured in a post-absorptive (fasted), controlled setting. Resting metabolic rate (RMR) is measured under less strict testing conditions. In clinical literature and software tools, the two terms are often used interchangeably, and equation outputs are frequently labeled either way. Neither BMR nor RMR represents total daily calorie needs. Basal metabolic rate excludes exercise, walking, work, digestion, and daily movement. Because it excludes these activities, a resting metabolic number represents only baseline survival expenditure rather than total daily energy needs. Total daily calorie needs are estimated by multiplying a resting estimate by an activity factor. A resting calculation provides the foundation, while activity factors scale that number to reflect physical movement. If you want to understand how resting metabolic rates can vary, read our guide on slow metabolism.

How Accurate Basal Metabolic Rate Estimates Are

The Mifflin-St Jeor equation provides an unbiased estimate across broad groups of adults, but predictive accuracy varies across individual body types. According to a 2013 validation study in non-obese and obese adults published in Clinical Nutrition, the Mifflin-St Jeor equation was unbiased, with a 95% confidence interval (CI) of -26 to +8 kcal/day. However, the researchers observed that predictive accuracy was lower in obese than non-obese volunteers regardless of the equation evaluated. Mathematical formulas rely on statistical averages derived from population data. The equation produces an estimate rather than a direct physical measurement of your specific tissues.

Measuring Versus Estimating

The only way to directly measure resting energy use is a resting metabolic rate test using indirect calorimetry. Mathematical formulas only approximate metabolic rate using statistical models. To see how a resting metabolic rate test directly measures energy expenditure using indirect calorimetry, read our guide to the resting metabolic rate test. Consumer technology companies offer alternative estimation tools. Smart scales produced by brands such as Renpho, Withings, and Tanita build a metabolic age number from an estimated BMR derived from bioelectrical impedance compared against population averages. These commercial devices come with significant limitations. The proprietary formulas powering smart scales are not published. Furthermore, no major medical body has validated metabolic age as a biomarker. If you encounter this metric on a home device, see our guide on metabolic age to understand how those numbers are constructed.

What a BMR Number Is Good For

A basal metabolic rate figure establishes the baseline energy required to sustain basic body functions while completely at rest. Because BMR excludes exercise, walking, work, daily movement, and digestion, it does not show how many calories you burn throughout a normal day. Instead, a resting estimate serves as the starting point for calculating total daily energy needs. Total daily needs are estimated by multiplying a resting estimate by an activity factor. Knowing your resting baseline prevents you from mistaking minimum survival requirements for total daily calorie expenditure.

What to Look For in a Body-Composition Scale

A bioimpedance scale trends body fat and muscle rather than just weight — useful when the number stalls but your composition is improving. The absolute body-fat % isn't lab-accurate, so use it for TRENDS at a consistent time of day. Choose one that syncs to an app so you can watch the trend line.

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