BMR Calculator
Calculate basal metabolic rate and resting calories
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
Why BMR Is a Starting Point, Not a Complete Metabolic Picture
BMR alone should never be used to set a diet target — it tells you what your body burns doing nothing, not what it burns living your actual life. Using BMR as a calorie target (rather than TDEE, which factors in activity) puts almost anyone into an unintentionally severe deficit, which is one of the more common mistakes people make when they encounter a BMR number without understanding what it represents.
The "slow metabolism" explanation for weight gain is largely a myth in the way it's commonly used — genuine metabolic differences between individuals of similar size are real but modest, while the actual adaptive thermogenesis that occurs during sustained dieting (a real, measurable 15-20% drop in BMR beyond what weight loss alone explains) is a different and more nuanced phenomenon than a permanently "broken" metabolism.
Formula-based BMR estimates carry a real margin of error (roughly ±10%) even though Mifflin-St Jeor is the most accurate of the commonly used equations. For clinical precision — useful for elite athletes or anyone with a suspected metabolic or thyroid issue — indirect calorimetry, which directly measures oxygen consumption, is the gold-standard alternative worth discussing with a doctor.
?Frequently Asked Questions
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.
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.
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.
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.
Genuine metabolic differences between similarly-sized individuals exist but are typically modest — usually within 5-10% of each other, not enough alone to explain large weight differences. What is well documented is adaptive thermogenesis: BMR can drop 15-20% beyond what weight loss alone would predict during sustained, aggressive dieting, which is a real and measurable phenomenon, distinct from the common but oversimplified idea of a permanently "broken" metabolism.
BMR is fairly stable over short periods (weeks) for someone at a stable weight, but it does shift gradually with sustained changes in body weight, muscle mass, and age. Significant weight loss or gain, a meaningful change in muscle mass from training, illness, or hormonal changes (including thyroid conditions) can all move BMR by a noticeable amount. Recalculating every few months, or after a significant weight change, keeps the estimate relevant.
Generally not recommended without medical supervision. Eating below BMR means your body isn't even getting enough energy to run basic functions like organ activity and temperature regulation from food alone, which tends to accelerate muscle loss, increase fatigue, and trigger a stronger adaptive metabolic slowdown than a moderate deficit would. Most sports nutrition guidelines recommend a deficit relative to TDEE (typically 300-500 kcal/day), staying at or above BMR, rather than dieting below it.