What is Resting Metabolic Rate?
Resting Metabolic Rate (RMR) is the total number of calories your body burns while at rest to maintain essential physiological functions such as breathing, circulating blood, regulating body temperature, growing and repairing cells, and maintaining organ function. It represents the largest component of total daily energy expenditure (TDEE), typically accounting for 60–75% of all calories burned each day.
RMR is measured under less restrictive conditions than Basal Metabolic Rate (BMR) and is therefore slightly higher. While laboratory measurement via indirect calorimetry is the gold standard, validated prediction equations provide reasonably accurate estimates for clinical and personal use.
BMR vs. RMR: What's the Difference?
| Parameter | BMR | RMR |
|---|---|---|
| Measurement conditions | Complete rest, dark room, post-absorptive state (12-hour fast), thermoneutral environment | Resting conditions, 3–4 hour fast, comfortable environment |
| Subject state | Awake but completely still, immediately after waking | Awake, resting quietly, may have been active earlier in the day |
| Typical difference | RMR is approximately 10–20% higher than BMR | |
| Clinical use | Research settings | Clinical practice and nutritional counseling |
In practice, the terms BMR and RMR are often used interchangeably, and the prediction equations most people use (Mifflin-St Jeor, Harris-Benedict) technically estimate RMR rather than true BMR.
RMR Formulas Explained
Mifflin-St Jeor Equation (1990)
Considered the most accurate for most people. Recommended by the Academy of Nutrition and Dietetics.
Female: RMR = 10 × Weight(kg) + 6.25 × Height(cm) − 5 × Age(y) − 161
Harris-Benedict Equation (Revised 1984)
Originally published in 1919, revised by Roza and Shizgal in 1984. Slightly less accurate than Mifflin-St Jeor but still widely used.
Female: RMR = 447.593 + 9.247 × W(kg) + 3.098 × H(cm) − 4.330 × A(y)
Katch-McArdle Equation (1996)
Uses lean body mass instead of total weight, making it more accurate for individuals who know their body fat percentage, especially lean or athletic individuals.
where LBM = Weight × (1 − Body Fat % / 100)
Energy Expenditure Diagram
Understanding TDEE
Total Daily Energy Expenditure (TDEE) is the total number of calories you burn per day. It is calculated by multiplying RMR by an activity factor:
| Activity Level | Description | Multiplier |
|---|---|---|
| Sedentary | Little or no exercise, desk job | 1.2 |
| Lightly Active | Light exercise 1–3 days/week | 1.375 |
| Moderately Active | Moderate exercise 3–5 days/week | 1.55 |
| Very Active | Hard exercise 6–7 days/week | 1.725 |
| Extra Active | Very hard exercise, physical job, or training twice/day | 1.9 |
Factors Affecting Metabolic Rate
- Lean body mass: The single most important determinant. Muscle tissue is metabolically active, burning ~6 kcal/lb/day at rest, compared to ~2 kcal/lb/day for fat tissue.
- Age: RMR decreases approximately 1–2% per decade after age 20, primarily due to loss of lean mass (sarcopenia) rather than aging per se.
- Sex: Males typically have 5–10% higher RMR than females of the same weight and age, due to greater lean mass and the effects of testosterone.
- Thyroid function: Thyroid hormones (T3, T4) are primary regulators of metabolic rate. Hypothyroidism reduces RMR by 15–40%, while hyperthyroidism increases it.
- Body temperature: For every 0.5°C increase in core body temperature, RMR increases approximately 7%.
- Caloric restriction: Prolonged caloric deficit causes "adaptive thermogenesis" — a decrease in RMR beyond what is expected from weight loss alone. This can persist for months or even years.
- Caffeine & stimulants: Caffeine can temporarily increase RMR by 3–11%, with the effect lasting 2–3 hours.
- Environmental temperature: Cold exposure activates brown adipose tissue and increases metabolic rate for thermogenesis.
Worked Example
A 30-year-old male, 70 kg, 170 cm, using Mifflin-St Jeor:
RMR = 700 + 1062.5 − 150 + 5 = 1,617.5 kcal/day
If this person is moderately active (multiplier 1.55):
To lose weight at a safe rate of ~0.5 kg/week, they would target approximately 2,007 kcal/day (500 kcal deficit).
Frequently Asked Questions
Which formula should I use?
For most people, the Mifflin-St Jeor equation is recommended as it has been shown to be the most accurate within ±10% of measured RMR for the general population. If you know your body fat percentage accurately (from DEXA, hydrostatic weighing, or BodPod), the Katch-McArdle formula may be more accurate, especially if you are very lean or very muscular.
How accurate are these formulas?
Prediction equations are typically accurate to within ±10–15% of measured RMR. Individual variation can be significant. Factors such as genetics, thyroid function, and metabolic adaptation from dieting can cause actual RMR to differ from predicted values. For precise measurement, indirect calorimetry in a clinical setting is the gold standard.
Does exercise increase my RMR?
Yes, both acutely and chronically. After intense exercise, Excess Post-Exercise Oxygen Consumption (EPOC) elevates metabolic rate for 12–48 hours. Chronically, resistance training increases lean mass, which permanently raises RMR. Each kilogram of muscle gained adds approximately 13 kcal/day to your resting energy expenditure.
Will eating too little slow my metabolism?
Yes. Prolonged caloric restriction causes adaptive thermogenesis, where your body reduces RMR to conserve energy. This reduction can be 5–15% beyond what would be expected from weight loss alone. This is one reason why gradual, moderate caloric deficits (300–500 kcal/day) are preferred over very low calorie diets.
How does body composition affect RMR?
Lean mass is the primary driver of RMR. Two people of the same weight can have very different RMR values if they have different amounts of muscle vs. fat. This is why the Katch-McArdle formula, which uses lean body mass, can be more accurate for athletes or very lean individuals.