BMR Calculator
Calculate basal metabolic rate and resting calories
What is BMR (Basal Metabolic Rate)?
Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest to maintain vital functions — breathing, circulation, cell production, temperature regulation, and organ function. BMR accounts for approximately 60–70% of total daily calorie expenditure in sedentary individuals and represents the minimum energy your body requires to survive.
BMR is calculated using validated equations that factor in weight, height, age, and sex. This calculator uses the Mifflin-St Jeor equation (1990), which is considered the most accurate predictor of resting metabolic rate for the general population according to the Academy of Nutrition and Dietetics — more accurate than the older Harris-Benedict equation (1919) it largely replaced.
BMR is the foundation for calculating TDEE (Total Daily Energy Expenditure). To find your maintenance calories, BMR is multiplied by an activity factor. Understanding your BMR helps you set accurate calorie targets for weight loss, maintenance, or muscle gain without guesswork.
How to Use This Calculator
Factors That Affect BMR
- • Muscle mass: More muscle = higher BMR (muscle burns ~3× more than fat at rest)
- • Age: BMR declines with age due to sarcopenia (muscle loss)
- • Thyroid function: Hypo/hyperthyroidism significantly affects BMR
- • Body temperature: Fever raises BMR ~7% per 0.5°C temperature increase
- • Hormones: Testosterone and estrogen levels influence metabolic rate
?Frequently Asked Questions
What is the difference between BMR and RMR?
BMR (Basal Metabolic Rate) is measured under strict laboratory conditions — fasted for 12+ hours, completely at rest, in a thermoneutral environment. RMR (Resting Metabolic Rate) is measured under less strict conditions and is typically 10–20% higher than true BMR. In practice, most calorie calculators — including this one — calculate RMR but label it as BMR. The two terms are often used interchangeably.
Why does BMR decrease when you diet?
When you reduce calorie intake, your body adapts by lowering its metabolic rate — a phenomenon called adaptive thermogenesis. This is an evolutionary survival mechanism. Studies show that after significant caloric restriction, BMR can decrease by 15–20% beyond what is expected from weight loss alone. This is why diets become progressively less effective over time and why refeeding periods and diet breaks are increasingly recommended in sports nutrition.
Does exercise increase BMR?
Yes, in two ways. First, resistance training builds muscle mass, and since muscle tissue burns more calories at rest than fat tissue, more muscle permanently elevates BMR. Second, intense exercise causes EPOC (Excess Post-exercise Oxygen Consumption) — elevated calorie burning for hours after a workout. Cardio alone has minimal lasting effect on BMR, which is why resistance training is the most effective long-term metabolic strategy.
How accurate is the Mifflin-St Jeor formula?
The Mifflin-St Jeor equation predicts resting metabolic rate within ±10% for most people. A 2005 study in the Journal of the American Dietetic Association found it to be the most accurate of four commonly used equations. However, it has higher error rates for obese individuals and very lean athletes. For clinical precision, indirect calorimetry (measuring oxygen consumption) is the gold standard.