Your Basal Metabolic Rate (BMR) is the number of calories your body burns to sustain basic physiological functions at complete rest — breathing, circulation, cell repair, and thermoregulation.
BMR represents 60–75% of most people's total daily calorie expenditure. Multiply it by an activity factor to get your TDEE (Total Daily Energy Expenditure) — the foundation for all calorie targets.
The Mifflin-St Jeor Equation
Developed in 1990, the Mifflin-St Jeor equation is the most validated BMR formula for the modern general population. A landmark 2005 meta-analysis of 47 studies found it to be within 10% accuracy for 82% of non-obese individuals. The older Harris-Benedict equation (1919) systematically overestimates BMR by 5–15% compared to measured values.
BMR vs RMR: What's the Difference?
BMR (Basal Metabolic Rate) is measured under strict laboratory conditions: fasted state, lying completely still, controlled temperature, awake but at complete rest. RMR (Resting Metabolic Rate) is measured under less strict conditions and is typically 10–20% higher. Most online calculators estimate RMR, though the terms are used interchangeably in practice.
Why BMR Decreases with Age
BMR declines approximately 2–3% per decade after age 20, primarily due to loss of muscle mass (sarcopenia). Muscle tissue burns roughly 13 kcal/kg/day at rest versus 4.5 kcal/kg/day for fat tissue. Resistance training can significantly slow age-related BMR decline by preserving and building muscle mass.
Factors That Affect Your BMR
Beyond age, sex, and body size, several factors influence BMR: body composition (more muscle = higher BMR), hormonal status (thyroid hormones directly regulate metabolic rate), temperature (BMR increases slightly in cold environments), pregnancy and breastfeeding (significantly elevated), and illness (infection raises BMR 7–13% per degree Celsius of fever).
💡 Expert Tips
- ✓Your BMR is your minimum — never eat below this for extended periods without medical supervision.
- ✓Muscle mass is the biggest modifiable driver of BMR. Resistance training 3× per week protects and builds metabolically active tissue.
- ✓Thyroid function significantly affects BMR — if your calculated BMR seems very different from your experience, consider a thyroid panel.
- ✓BMR decreases ~3% per decade — account for this when setting calorie targets as you age.
- ✓Recalculate your BMR any time your weight changes by 5+ kg.
📊 BMR Reference Ranges by Age and Sex
| Age Group | Male BMR | Female BMR | Notes |
|---|---|---|---|
| 18–25 | 1,750–1,900 kcal | 1,450–1,600 kcal | Peak metabolic rate |
| 26–35 | 1,700–1,850 kcal | 1,400–1,550 kcal | Slight decline begins |
| 36–45 | 1,620–1,780 kcal | 1,350–1,500 kcal | ~3% reduction/decade |
| 46–55 | 1,560–1,720 kcal | 1,300–1,450 kcal | Hormonal changes factor in |
| 56–65 | 1,480–1,650 kcal | 1,250–1,390 kcal | Sarcopenia accelerates |
| 66–75 | 1,400–1,580 kcal | 1,190–1,340 kcal | Resistance training helps |
| 76+ | 1,310–1,510 kcal | 1,090–1,280 kcal | High individual variation |