Mifflin-St Jeor BMR Calculator

The Mifflin-St Jeor equation traces back to a single landmark paper: Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. “A new predictive equation for resting energy expenditure in healthy individuals,” published in the American Journal of Clinical Nutrition in February 1990 (volume 51, pages 241-247, PMID 2305711). The research team recruited 498 healthy adults from the Reno, Nevada area: 251 women and 247 men, aged 19 to 78, with body mass index ranging from 17.4 to 42.4 kg/m². The cohort intentionally spanned both lean and obese subjects so the equation would generalize across populations rather than fitting only one body type.

Each participant underwent gold-standard indirect calorimetry. After a 12-hour overnight fast, subjects rested supine for 30 minutes in a thermoneutral room before measurement. Resting energy expenditure (REE, often used interchangeably with BMR for adult populations) was measured for 40 minutes using a ventilated hood and a Beckman metabolic cart that continuously sampled oxygen consumption (VO2) and carbon dioxide production (VCO2). The Weir equation was then applied to convert gas exchange into kilocalories per day. This is the same protocol still used by clinical metabolic carts today.

Mifflin and colleagues then fit a multiple linear regression with measured REE as the dependent variable and weight (kg), height (cm), age (years), and sex as predictors. The model that emerged was elegant: BMR = 10 x weight + 6.25 x height - 5 x age, with a final +5 constant for men or -161 constant for women. The model explained roughly 71% of the variance in measured REE (R-squared 0.71) and the standard error of the estimate was 96 kcal/day. When tested against the five most commonly used predictive equations of the time (Harris-Benedict 1919, FAO/WHO 1985, Owen 1986/1987, Schofield 1985, and Liu 1995), Mifflin-St Jeor produced lower mean error and a tighter prediction interval.

In 2005, Frankenfield, Roth-Yousey, and Compher published a systematic review in the Journal of the American Dietetic Association comparing predictive BMR equations head-to-head against indirect calorimetry across multiple cohorts. Their conclusion: the Mifflin-St Jeor equation predicted resting metabolic rate within +/- 10% of measured values in approximately 82% of non-obese adults and 70% of obese adults, performing better than Harris-Benedict, Owen, and WHO equations. On the basis of that review the Academy of Nutrition and Dietetics (then the American Dietetic Association) recommended Mifflin-St Jeor as the preferred predictive equation for healthy adults in clinical practice. That endorsement is why today every registered dietitian, hospital nutrition team, and clinical software package defaults to the Mifflin-St Jeor formula when indirect calorimetry is not available.

For the muscular minority of the population (lifters, athletes, bodybuilders), Mifflin tends to underestimate BMR by 5-10% because it uses total body weight rather than lean body mass. That is why this calculator also implements Katch-McArdle and Cunningham — two lean-mass-based equations — so muscular users can compare. For the general population, however, Mifflin-St Jeor remains the most reliable predictor of BMR available without a metabolic cart.