BMR Basal Metabolic Rate Calculator

Basal metabolic rate (BMR) is the quantity of energy expressed in kilocalories that a human body requires at complete rest over a 24-hour period to sustain vital organ function in a thermoneutral environment and post-absorptive state. This calculator implements two widely cited predictive formulas in parallel: the Mifflin-St Jeor equation published in 1990, and the revised Harris-Benedict equations published by Roza and Shizgal in 1984. Both formulas accept gender, age in years, standing height in centimeters, and body weight in kilograms as inputs. The two results are displayed side by side to illustrate the variation between standard predictive models for the same individual parameters.

BMR Formulas, Coefficients, and Standard Interpretation

Both formulas implemented by this calculator are multiple linear regression equations predicting basal or resting metabolic rate in kilocalories per day from four independent variables. The Mifflin-St Jeor equation imposes the constraint that the partial regression coefficients for weight, height, and age are identical across genders with only a single gender-specific additive constant separating the two. The revised Harris-Benedict equation estimates gender-specific coefficients for all four predictor variables, yielding a more flexible functional form but requiring four constants per gender rather than one.

Mifflin-St Jeor Equation (1990)

The Mifflin-St Jeor equation was developed from indirect calorimetry measurements on 498 subjects (247 male, 251 female) spanning an age range of 19 to 78 years and a weight range of 35.4 to 141.4 kilograms. The authors explicitly constrained the three slope coefficients to be equal across genders and tested the hypothesis that such a constrained model did not perform worse than the fully gender-specific model on their data. The equations are: Male BMR = (10 × weight_kg) + (6.25 × height_cm) − (5 × age_years) + 5. Female BMR = (10 × weight_kg) + (6.25 × height_cm) − (5 × age_years) − 161. The 166 kcal/day difference between the male and female intercepts is the only gender-specific term in the entire system.

Revised Harris-Benedict Equation (Roza & Shizgal, 1984)

Harris and Benedict's original 1918/1919 equations were based on a sample of 136 men and 103 women measured predominantly in the northeastern United States. By the 1980s, multiple researchers observed that the original coefficients produced systematic biases on contemporary populations. Roza and Shizgal assembled a dataset of 239 measured BMRs (107 men, 112 women) collected via indirect calorimetry under strict basal conditions between 1974 and 1982 at institutions in Montreal and Boston, then re-estimated the Harris-Benedict functional form using ordinary least squares regression. Male BMR = 88.362 + (13.397 × weight_kg) + (4.799 × height_cm) − (5.677 × age_years). Female BMR = 447.593 + (9.247 × weight_kg) + (3.098 × height_cm) − (4.330 × age_years). All four coefficients differ between genders.

What BMR Represents (Definition Only)

The term basal metabolic rate denotes the rate of energy utilization by an organism under the specific measurement conditions of complete muscular relaxation, mental repose, thermoneutral ambient temperature (approximately 20–26 °C for clothed adults), and a minimum of 12 hours after the last caloric intake so that the digestive tract is in the post-absorptive state and the specific dynamic action of food does not contribute to measured heat production. BMR is conventionally expressed in kilocalories per day for human nutritional contexts even though the formal SI unit of metabolic rate is watts (joules per second). The analogous quantity measured after briefer rest intervals without the full post-absorptive requirement is termed resting metabolic rate (RMR), which is typically 3–10 percent numerically higher than BMR for the same individual.

Common Misconceptions About BMR Formulas

Several interpretive points recur in discussions of predictive BMR equations. First, all formulas of this class produce population-level estimates; individual metabolic rates measured by indirect calorimetry routinely differ from equation predictions by ±10 percent or more even within the validation samples. Second, the coefficients of these equations were estimated on generally healthy adult populations and their accuracy is not established for adolescents, pregnant individuals, individuals with acute or chronic metabolic conditions, or individuals at the extremes of the body composition or age ranges. Third, the numerical difference between the Mifflin and revised Harris-Benedict outputs for the same inputs is not an indicator of error in either formula but rather reflects the distinct datasets and regression strategies underlying the two models. Fourth, neither formula accounts for body composition independent of total weight, so individuals with identical height, weight, age, and gender but substantially different ratios of lean mass to adipose mass receive identical BMR predictions despite potentially differing true metabolic rates.