Basal Metabolic Rate (BMR) is the number of calories your body burns to perform basic, life-sustaining functions such as breathing, blood circulation, cell production, nutrient processing, and body temperature regulation. It accounts for 60% to 75% of your total daily energy expenditure (TDEE).
BMR is measured under strict laboratory conditions: awake, lying still, thermally neutral environment, after 12 hours of fasting. Resting Metabolic Rate (RMR) is slightly less strict and usually 5% to 10% higher than BMR, though the terms are often used interchangeably in practice.
Equations used to estimate BMR
The Mifflin-St Jeor equation is widely considered the most accurate clinical formula for estimating BMR. For men: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) + 5. For women: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) - 161.
The Katch-McArdle equation is even more accurate for individuals who know their body fat percentage, as lean body mass is the primary driver of metabolic activity: BMR = 370 + (21.6 x Lean Body Mass in kg). Muscle tissue consumes roughly 13 kcal/kg per day at rest, compared to fat tissue which consumes roughly 4.5 kcal/kg per day.
Factors that influence your BMR
BMR decreases with age at a rate of roughly 1% to 2% per decade after age 30, primarily due to loss of muscle mass (sarcopenia) and hormonal shifts. Height and weight also play major roles: larger bodies have more metabolically active tissue and higher surface area, requiring higher energy expenditure to maintain body temperature.
Prolonged severe calorie restriction causes adaptive thermogenesis (often called metabolic adaptation), where the body slows BMR by up to 10-15% to conserve energy. Resistance training and adequate protein intake are the most effective ways to preserve lean muscle mass and maintain BMR during weight loss.