What Resting Metabolic Rate Is
Resting Metabolic Rate (RMR) is the number of calories your body burns per day while at rest — no exercise, no movement beyond the baseline needed to sustain life. It represents the energy cost of breathing, keeping your heart beating, maintaining body temperature, performing cellular repair, synthesising proteins, and running all the biochemical processes that keep you alive.
RMR accounts for approximately 60–70% of total daily energy expenditure (TDEE) in sedentary individuals. The remaining energy goes to physical activity (including all intentional exercise and non-exercise activity thermogenesis, or NEAT) and the thermic effect of food (the energy cost of digesting and metabolising what you eat, typically 8–10% of caloric intake).
Understanding your RMR provides the baseline for estimating your total caloric needs, creating a calorie deficit for fat loss, or calculating a surplus for muscle gain. Without an accurate RMR estimate, calorie targets are based purely on guesswork.
RMR vs BMR: What’s the Difference?
The terms Resting Metabolic Rate (RMR) and Basal Metabolic Rate (BMR) are often used interchangeably in practice, but they have different technical definitions:
BMR is measured under strictly controlled conditions: complete rest in a recumbent position, measured in the morning after an overnight fast, in a thermoneutral environment (neither cold nor warm). These conditions minimise all energy expenditure beyond the absolute minimum required to sustain life.
RMR is measured under less strict conditions. The person is still at rest but may be seated rather than recumbent, and the thermoneutral environment requirement may be relaxed. RMR is therefore slightly higher than BMR — typically by 5–15%.
In everyday usage, most “BMR calculators” actually compute what would more accurately be called RMR. The distinction matters most in clinical research; for practical nutrition planning, the difference is small enough that both terms are useful interchangeably.
The Mifflin-St Jeor Equation
The Mifflin-St Jeor equation was published in 1990 by Mark Mifflin and Sachiko St Jeor and colleagues at the University of Nevada. It was derived from indirect calorimetry measurements in 498 healthy adults aged 19–78 (251 male, 247 female), using body weight, height, age, and sex as predictors.
The equations are:
Male: RMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
Female: RMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
The sex constant difference is 166 kcal/day (5 − (−161) = 166), reflecting the average metabolic difference between males and females of the same size and age.
Why Mifflin-St Jeor Is Preferred
Multiple systematic reviews and meta-analyses have evaluated the accuracy of RMR prediction equations. A 2005 comparison by Frankenfield et al. (Journal of the American Dietetic Association) found that the Mifflin-St Jeor equation predicted measured RMR within 10% of actual values in approximately 82% of subjects — a higher accuracy rate than Harris-Benedict (approximately 50–60% within 10%) or other common equations.
The Harris-Benedict equation, published in 1919 and widely used for decades, was derived from a much smaller sample using less precise measurement methods. The 1984 revision by Roza and Shizgal improved it somewhat, but it still consistently overestimates RMR in overweight individuals. Mifflin-St Jeor performs more consistently across a range of body weights.
Worked Example: Male, 30 Years, 175 cm, 70 kg
Applying the Mifflin-St Jeor equation:
RMR = (10 × 70) + (6.25 × 175) − (5 × 30) + 5
= 700 + 1093.75 − 150 + 5
= 1648.75 → 1649 kcal/day
This represents the number of calories this person burns per day at complete rest. To estimate total daily needs, multiply by an activity factor (see TDEE calculation below).
Female at the same measurements (163 cm, 60 kg, 35 years):
RMR = (10 × 60) + (6.25 × 163) − (5 × 35) − 161 = 600 + 1018.75 − 175 − 161 = 1282.75 → 1283 kcal/day
The Cunningham Formula: Better for Athletes
The Cunningham formula (1980) takes a different approach: instead of predicting RMR from total body weight, it predicts it from lean body mass (also called fat-free mass). The equation is:
RMR = 500 + 22 × lean body mass (kg)The rationale is straightforward: fat tissue is metabolically relatively inactive compared to muscle, organ, and connective tissue. If two people have the same total weight but one has more muscle and less fat, the more muscular person will have a higher RMR. The Mifflin-St Jeor equation accounts for this only indirectly through total weight; the Cunningham formula addresses it directly.
For athletes with high muscle mass relative to their weight, Cunningham generally provides a better RMR estimate. For average adults without precise lean body mass data, Mifflin-St Jeor is more practical because it does not require body composition measurement.
Example: male, 70 kg, lean body mass = 57 kg (body fat ≈ 18.6%)
Cunningham RMR = 500 + (22 × 57) = 500 + 1254 = 1754 kcal/day
Compare to Mifflin-St Jeor at the same weight/height/age: 1649 kcal/day.
The Cunningham estimate is higher, reflecting the high lean mass relative to total weight. For a highly muscular individual, this higher estimate is likely to be closer to their measured RMR.
From RMR to Total Daily Calories (TDEE)
RMR is only one component of total energy expenditure. To estimate your total daily caloric needs, multiply RMR by an activity factor:
| Activity level | Factor | Description |
|---|---|---|
| Sedentary | 1.2 | Desk job, little or no exercise |
| Lightly active | 1.375 | Light exercise 1–3 days/week |
| Moderately active | 1.55 | Moderate exercise 3–5 days/week |
| Very active | 1.725 | Hard exercise 6–7 days/week |
| Extremely active | 1.9 | Hard exercise + physical job |
These multipliers (commonly called the Harris-Benedict activity factors) are rough guides. Individual variation in NEAT — spontaneous activity like fidgeting, posture changes, and walking around — means that actual TDEE can differ from the formula estimate by 15–25%.
The TDEE calculator on this site applies these activity factors directly to your RMR estimate.
Limitations of Prediction Equations
All prediction equations carry uncertainty. The Mifflin-St Jeor equation has a reported 95% prediction interval of approximately ±300 kcal/day for individuals — meaning the true RMR may be 300 kcal/day above or below the prediction in any given person. Across a population, the equation performs well on average; for any individual, the error can be substantial.
Factors that prediction equations do not account for include:
- Body composition: Muscle and fat have different metabolic rates; two people at the same weight with different compositions will have different RMRs
- Hormonal status: Hypothyroidism significantly reduces RMR; hyperthyroidism raises it; other hormonal conditions affect energy metabolism
- Chronic illness: Various medical conditions alter metabolic rate
- Medications: Some drugs (beta-blockers, corticosteroids, certain antidepressants) affect metabolic rate
- Adaptation to caloric restriction: Prolonged caloric restriction can reduce RMR beyond what weight loss alone would predict (adaptive thermogenesis)
Direct measurement via indirect calorimetry (measuring exhaled gas composition) is the gold standard for RMR determination and is available at some sports performance clinics, hospitals, and research settings.
Frequently Asked Questions
Can I increase my RMR? Building muscle mass increases RMR because muscle tissue is more metabolically active than fat tissue. A kilogram of muscle burns approximately 13 kcal/day at rest, while a kilogram of fat burns approximately 4.5 kcal/day. Resistance training over months to years can increase lean mass and thereby raise RMR. The magnitude of this effect in practice is often smaller than expected — adding 5 kg of muscle would increase RMR by approximately 65 kcal/day.
Does eating more frequently boost metabolism? There is no consistent evidence that eating frequency affects RMR. The thermic effect of food (energy spent on digestion) is proportional to the total calories consumed, not to the number of meals. Eating 6 small meals vs 3 larger meals has the same thermic effect if total calories are equal.
What happens to RMR as we age? RMR declines with age, primarily because muscle mass tends to decrease with age (sarcopenia) and because some organ metabolic rates slow down. Studies estimate a decline of approximately 1–2% per decade after age 20, though this is highly variable. The Mifflin-St Jeor equation partially accounts for age through the −5×age term, though this linear correction is an approximation of a more complex relationship.