What Is Basal Metabolic Rate, and Which Formula Should You Use?

Basal metabolic rate (BMR) is the energy your body uses at complete rest, awake, to keep you alive: breathing, circulation, keeping warm, and the work of your organs. For most people it’s the largest part of the energy they use each day, often 60% to 70% of it. Few people ever have it measured, so it’s usually estimated from an equation, and the equations disagree by up to a couple of hundred kilocalories a day.

This article is general information, not medical or nutritional advice.

How It’s Measured

BMR is measured by indirect calorimetry: after an overnight fast and rest, you breathe into a hood or mask while a machine measures how much oxygen you use and carbon dioxide you make. Measured strictly, it’s BMR; measured under looser conditions, it’s called resting metabolic rate (RMR), usually slightly higher. The equations below were built from such measurements.

The Three Common Formulas

• Mifflin-St Jeor (1990). Uses weight, height, age, and sex: 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years), plus 5 for men or minus 161 for women. It was built from 498 healthy adults, including people with obesity.
• Harris-Benedict (1918, revised 1984). The oldest, from a study of 239 people at the Carnegie Institution, using the same inputs. Roza and Shizgal refitted its coefficients in 1984 with more data.
• Katch-McArdle. Uses only lean body mass: 370 + 21.6 × lean mass (kg), where lean mass is your weight minus your fat. It ignores age and sex, on the basis that lean tissue drives most of the energy you use at rest. It’s only as good as your body fat measurement, which from a smart scale can be several points off.

Which One to Use

In a 2005 systematic review for the American Dietetic Association, Frankenfield and colleagues compared the common equations against measured resting metabolic rate and found Mifflin-St Jeor the most reliable, predicting within 10% of measured RMR in more non-obese and obese adults than the others. Harris-Benedict tended to overestimate. Even the best equation was off by more than 10% for some people, more often in older adults and some ethnic groups, which were less well represented in the studies.

For most people, Mifflin-St Jeor is a reasonable default. Katch-McArdle can suit someone with an accurate body fat measurement and an unusual build, such as a very muscular athlete, though there’s little research comparing it with the others.

From BMR to What You Need

BMR is not your daily energy need. Your total adds the energy of digesting food and all your movement, from exercise to fidgeting, and differs a lot from person to person. The Dietary Reference Intakes’ equations for estimated energy requirements include physical activity directly. Your weight’s trend over weeks is the best check of whether your intake matches what you use.

Following It in Kaizora

Basal Metabolic Rate is a derived metric on the Body page. Choose its formula in Settings, under General, or from the metric’s own settings menu: Mifflin-St Jeor and Harris-Benedict use your weight, height, sex, and age (from your date of birth), and Katch-McArdle your weight and body fat percentage. Changing the formula recalculates your whole history, so a chart never mixes formulas. The metric’s About section says how it’s calculated.

The Short Version

• BMR is the energy you use at complete rest, most of your daily energy use.
• Mifflin-St Jeor is the best-supported equation for most adults.
• Any equation can be 10% or more off for an individual.
• Your weight’s trend tells you more than any formula about your energy balance.

Sources

• Mifflin MD et al., “A new predictive equation for resting energy expenditure in healthy individuals,” American Journal of Clinical Nutrition 51, no. 2 (1990): 241–247. doi:10.1093/ajcn/51.2.241
• Harris JA, Benedict FG, “A biometric study of human basal metabolism,” Proceedings of the National Academy of Sciences 4, no. 12 (1918): 370–373. doi:10.1073/pnas.4.12.370
• Roza AM, Shizgal HM, “The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass,” American Journal of Clinical Nutrition 40, no. 1 (1984): 168–182. doi:10.1093/ajcn/40.1.168
• Frankenfield D, Roth-Yousey L, Compher C, “Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review,” Journal of the American Dietetic Association 105, no. 5 (2005): 775–789. doi:10.1016/j.jada.2005.02.005