BMR Calculator

Basal metabolic rate is the energy an organism spends at complete rest, in a thermoneutral and post-absorptive state, to run the processes that keep it alive: ion pumping across cell membranes, protein turnover, cardiac and respiratory work, and hepatic and renal function. In adults it accounts for roughly 60 to 75 percent of total daily energy expenditure, with activity thermogenesis and the thermic effect of food making up the rest. Measured properly by indirect calorimetry, it is inferred from oxygen consumption and carbon dioxide production. The equations offered here are regressions fitted to such measurements. Mifflin-St Jeor uses 10 kcal per kilogram of body mass, 6.25 per centimeter of height and minus 5 per year of age, with a constant of plus 5 for men and minus 161 for women; the age term captures the gradual loss of metabolically active tissue. Katch-McArdle takes a different route, estimating 370 kcal plus 21.6 kcal per kilogram of fat-free mass, which is why it requires a body fat percentage and why it suits lean or heavily muscled people whose total weight misleads the other formulas. Typical adult results run about 1,300 to 1,900 kcal per day. All these equations carry a standard error near 10 percent, so roughly one person in five falls outside that band, and none of them are validated for pregnancy, severe obesity, or critical illness.

Measurement of animal heat production began with Antoine Lavoisier and Pierre-Simon Laplace, who around 1782 placed a guinea pig in an ice calorimeter and showed that the heat it released melted ice in proportion to the carbon dioxide it exhaled, establishing respiration as a form of combustion. In the nineteenth century Max Rubner argued in 1883 that metabolic rate scales with body surface area, a rule that dominated clinical practice for decades before Max Kleiber proposed in 1932 that it scales closer to body mass raised to the three-quarter power. Human prediction equations arrived with James Arthur Harris and Francis Gano Benedict, whose 1919 Carnegie Institution of Washington monograph fitted regressions to respiration-calorimeter data from several hundred subjects; their formulas remained the standard for most of the century. Arthur Roza and Harry Shizgal revised the coefficients in 1984 after finding that the originals overestimated expenditure in modern populations. Mark Mifflin, Sachiko St Jeor and colleagues published a simpler and better-calibrated equation in the American Journal of Clinical Nutrition in 1990, derived from 498 healthy adults. A systematic review published by the American Dietetic Association in 2005 compared the competing equations against measured resting expenditure and concluded that Mifflin-St Jeor performed best for non-obese and obese adults alike, which is why it is the default here.