What Is BMR?
Basal Metabolic Rate (BMR) reflects the energy required to support essential physiological functions at complete rest, including breathing, circulation, brain activity, temperature regulation, and cellular processes. It represents the foundational caloric expenditure if an individual spent 24 hours awake in a thermoneutral environment without physical movement or digestion.
For most individuals, basal metabolism accounts for the largest proportion of total daily energy expenditure, typically representing roughly 60% to 75% of daily calorie burn. The remainder is divided between physical activity (both structured exercise and non-exercise daily movement) and the energetic cost of digesting nutrients.
Energy for involuntary organ functions and cellular turnover at rest.
Daily standing, walking, occupational tasks, and structured workouts.
Energy expended digesting, absorbing, and processing meals.
BMR vs. Resting Metabolic Rate (RMR)
BMR and RMR are closely related measures of resting energy expenditure, but they are defined under different testing protocols:
- BMR (Basal Metabolic Rate): Measured under standardized conditions immediately upon waking after 8 hours of sleep, following a 12-hour overnight fast, in a darkened, temperature-controlled room with zero prior physical movement.
- RMR (Resting Metabolic Rate): Measured under somewhat less restrictive clinical conditions (such as a clinic visit during the day) and may therefore be somewhat higher due to minor residual postural or digestive activity.
Most predictive mathematical formulas provide practical estimates of resting daily energy expenditure for general lifestyle planning.
How Is BMR Calculated?
Direct measurement of metabolic rate requires specialized clinical metabolic carts using indirect calorimetry. To estimate BMR without lab equipment, researchers have developed predictive equations based on biometric variables such as body weight, height, age, and biological sex.
1. The Mifflin-St Jeor Formula (1990)
Recommended DefaultIntroduced in 1990 by Dr. MD Mifflin and Dr. ST St Jeor, this is a commonly used general-purpose predictive equation for estimating resting energy expenditure in healthy adults.
Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
2. The Revised Harris-Benedict Formula (Roza & Shizgal, 1984)
A historical predictive equation originally developed by Harris and Benedict in 1919 and re-evaluated by Roza and Shizgal in 1984 using updated physiological datasets.
Men: BMR = 88.362 + (13.397 × weight in kg) + (4.799 × height in cm) − (5.677 × age in years)
Women: BMR = 447.593 + (9.247 × weight in kg) + (3.098 × height in cm) − (4.330 × age in years)
3. The Katch-McArdle Formula (Body Fat & Lean Mass)
Requires Body Fat %The Katch-McArdle equation calculates BMR directly from estimated Lean Body Mass (LBM) rather than total body weight. It can be useful for people who have a reasonably accurate body-fat estimate, such as athletes and individuals with known body composition.
Step 1 (Lean Mass): LBM (kg) = Total Weight (kg) × [1 − (Body Fat % ÷ 100)]
Step 2 (BMR): BMR = 370 + (21.6 × LBM in kg)
Note: Errors in the body fat measurement directly affect the resulting BMR estimate. It is not automatically more accurate for every person.
Metric vs. Imperial Unit Conversions
Standard equations calculate expenditure using metric units (kilograms and centimeters). If measuring in imperial units:
- Weight in kg = Weight in lbs ÷ 2.20462
- Height in cm = (Feet × 12 + Inches) × 2.54
Why Do Different BMR Calculators Give Different Results?
It is common to notice that different online calculators provide slightly different numbers for the same person. This occurs for several valid physiological and mathematical reasons:
- Different Population Datasets: Equations such as Mifflin-St Jeor, Harris-Benedict, and Schofield/WHO were developed using different demographic cohorts from different eras, with varying average heights, body compositions, and physical characteristics.
- Different Mathematical Variables: Weight-based equations (Mifflin-St Jeor) use total body weight, while composition-based equations (Katch-McArdle) use estimated lean body mass.
- All Calculations Are Estimates: A variance between formulas does not mean a calculator is broken. Mathematical equations provide statistical approximations; true resting energy expenditure can only be confirmed through laboratory testing.
BMR Calculator for Women
In population-based equations such as Mifflin-St Jeor, female BMR includes a sex-specific intercept constant of −161 to account for statistical baseline differences in body composition between sexes.
BMR = (10 × 60) + (6.25 × 165) − (5 × 30) − 161
BMR = 600 + 1031.25 − 150 − 161 = 1,320 kcal/day (estimated)
Menstrual Cycle Considerations
Hormonal changes can influence energy expenditure across the menstrual cycle, with some studies observing small fluctuations during the luteal phase. However, standard BMR equations do not adjust calculations for menstrual-cycle phase, and the formula output should be interpreted as a general baseline estimate.
BMR Calculator for Men
On average, men tend to have greater lean body mass than women of the same total body weight, which is one factor associated with differences in resting energy expenditure. Individual body composition varies considerably.
In the Mifflin-St Jeor equation, this demographic baseline difference is reflected by an intercept constant of +5:
BMR = (10 × 80) + (6.25 × 180) − (5 × 30) + 5
BMR = 800 + 1125 − 150 + 5 = 1,780 kcal/day (estimated)
BMR vs. TDEE: What's the Difference?
A fundamental concept in energy balance is distinguishing between BMR (resting metabolic baseline) and TDEE (total daily energy expenditure):
Common physical activity multipliers used to estimate TDEE from BMR include:
| Activity Level | Multiplier | Lifestyle Description |
|---|---|---|
| Sedentary | 1.200 | Desk occupation, minimal walking, little or no structured exercise |
| Lightly Active | 1.375 | Light exercise or brisk walking 1–3 days per week |
| Moderately Active | 1.465 | Moderate training or sports 4–5 days per week |
| Active | 1.550 | Daily exercise or intense sports 3–4 days per week |
| Very Active / Extreme | 1.725 | Intense exercise 6–7 days per week or physically demanding job |
Can You Use BMR to Lose Weight?
When planning weight changes, BMR is not the same as a daily calorie target. Calorie adjustments are planned relative to your estimated TDEE, which reflects your total energy expenditure including daily movement.
1. Weight Loss Deficit
Example targets often apply a modest adjustment of 250 to 500 kcal/day below estimated TDEE.
2. Weight Maintenance
Estimated maintenance calories correspond to calculated TDEE with no calorie adjustment.
3. Calorie Surplus
Example targets for weight gain or muscle support typically add 200 to 300 kcal/day above estimated TDEE.
How Accurate Are BMR Calculators?
Formula-based calculators estimate BMR, while clinical indirect calorimetry can directly measure resting energy expenditure under controlled laboratory conditions by measuring respiratory oxygen consumption ($VO_2$) and carbon dioxide production ($VCO_2$).
Dual-Energy X-Ray Absorptiometry (DEXA) measures body composition (bone mineral density, fat mass, and lean mass) and does not directly measure BMR, though body composition data can help inform composition-based estimates such as Katch-McArdle.
Predictive Equations Overview
| Equation | Description & Intended Context |
|---|---|
| Mifflin-St Jeor (1990) | Commonly used general-purpose predictive equation based on weight, height, age, and sex. |
| Revised Harris-Benedict (1984) | Historical predictive equation re-evaluated by Roza and Shizgal. |
| Katch-McArdle (1977 / 1991) | Calculates resting metabolism from estimated lean body mass; useful when reliable body composition data is available. |
| WHO / Schofield (1985) | Age- and sex-stratified cohort equations developed from international population datasets. |
Scientific Literature & Guideline Citations
Last reviewed: August 2026- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. "A new predictive equation for resting energy expenditure in healthy individuals." The American Journal of Clinical Nutrition, 1990; 51(2): 241–247.
- Roza AM, Shizgal HM. "The Harris Benedict equation reevaluated: resting energy expenditure and the protein requirement." The American Journal of Clinical Nutrition, 1984; 40(1): 168–182.
- Katch FI, McArdle WD. Nutrition, Weight Control, and Exercise. Houghton Mifflin, 1977; Cunningham JJ. "Body composition as a determinant of energy expenditure: a synthetic review and a proposed general prediction equation." The American Journal of Clinical Nutrition, 1991; 54(6): 963–969.
- 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, 2005; 105(5): 775–789.
- World Health Organization (WHO) / Schofield WN. "Predicting basal metabolic rate, new standards and review of previous work." Human Nutrition: Clinical Nutrition, 1985; 39 Suppl 1: 5–41.
- American College of Sports Medicine (ACSM). ACSM's Guidelines for Exercise Testing and Prescription, 11th Edition. Wolters Kluwer, 2021.