How to Use the BMR Calculator
Basal Metabolic Rate is the foundation of energy balance. Follow these steps:
- Select your unit preference: Choose Metric (centimeters and kilograms) or Imperial (inches and pounds).
- Input your age and biological sex: Age and sex heavily influence organ mass, bone density, and hormonal profile.
- Enter your height and weight: Provide accurate measurements taken in the morning.
- Examine baseline energy expenditure: The calculator outputs your 24-hour basal burn and compares Mifflin-St Jeor against the revised Harris-Benedict equation.
What is Basal Metabolic Rate (BMR)?
Your Basal Metabolic Rate (BMR) is the minimal rate of energy expenditure per unit of time by endothermic animals at rest. For humans, BMR represents the number of calories burned if you were to remain completely motionless in bed for 24 hours in a climate-neutral room while fasting.
In a typical sedentary adult, BMR accounts for 60% to 75% of total daily energy expenditure. The remainder is divided between the Thermic Effect of Food (TEF, ~10%) and Physical Activity / Exercise (PA/EAT/NEAT, 15%–30%).
Where Do Your Resting Calories Go?
Many people assume their skeletal muscles burn the majority of baseline calories. In reality, internal organs represent the vast majority of resting metabolic draw despite contributing a small fraction of body weight:
- Liver: ~27% of BMR (gluconeogenesis, detoxification, protein synthesis)
- Brain: ~19% of BMR (ion pumping, neurotransmitter synthesis)
- Skeletal Muscle: ~18% of BMR at complete rest
- Kidneys: ~10% of BMR (blood filtration and electrolyte balance)
- Heart: ~7% of BMR (continuous muscular pumping)
- Adipose & Other Tissues: ~19% of BMR
Formulas & Methodologies
The Mifflin-St Jeor Equation (1990)
Widely recognized as the most clinically reliable predictive equation:
Women: BMR = (10 × weight in kg) + (6.25 × height in cm) - (5 × age in years) - 161
Revised Harris-Benedict Equation (1984)
Originally formulated in 1919 and refined by Roza and Shizgal in 1984:
Women: BMR = 447.593 + (9.247 × kg) + (3.098 × cm) - (4.330 × age)
Step-by-Step Example Calculation
Let us calculate the BMR for a 30-year-old male, 180 cm tall, weighing 80 kg using Mifflin-St Jeor:
- Weight component:
10 × 80 = 800 - Height component:
6.25 × 180 = 1,125 - Age component:
5 × 30 = 150 - BMR =
800 + 1,125 - 150 + 5 = 1,780 kcal / day
Factors That Influence BMR
- Lean Muscle Mass: Adding muscle tissue increases BMR because muscle tissue is metabolically active, burning roughly 12–15 kcal/kg/day at rest compared to ~4.5 kcal/kg/day for body fat.
- Age: BMR declines by approximately 1% to 2% per decade after age 25, largely driven by age-related sarcopenia (loss of skeletal muscle mass). Resistance training helps mitigate this decline.
- Prolonged Calorie Restriction (Adaptive Thermogenesis): When you remain in a severe calorie deficit for months, thyroid hormones (T3) and leptin downregulate, temporarily lowering your BMR below mathematical predictions.
Frequently Asked Questions
BMR is measured under strict clinical laboratory conditions immediately upon waking after an overnight fast. Resting Metabolic Rate (RMR) is measured under less restrictive resting conditions and is typically 5% to 10% higher than BMR.
Do not diet directly down to your BMR. Instead, multiply your BMR by your physical activity factor to determine your Total Daily Energy Expenditure (TDEE), and create a moderate 300 to 500 kcal deficit from your TDEE using our TDEE Calculator.