Your Basal Metabolic Rate is the number of calories your body burns at complete rest, just to keep you alive. It's the baseline for understanding your daily calorie needs.
This is how many calories your body needs at complete rest. Your actual daily needs are higher based on your activity level.
This BMR calculator uses the Mifflin-St Jeor equation and is for informational purposes only. Consult a healthcare provider before making dietary changes.
Basal Metabolic Rate (BMR) is the energy your body burns at complete rest in a 24-hour period. The Mifflin-St Jeor equation, the most accurate for healthy adults, calculates: BMR (men) = 10×weight(kg) + 6.25×height(cm) − 5×age + 5; BMR (women) = 10×weight(kg) + 6.25×height(cm) − 5×age − 161. Eating below BMR for extended periods causes muscle loss and metabolic adaptation.
Source: Mifflin MD, St Jeor ST. Am J Clin Nutr 1990Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest, just to stay alive. It's the energy required to keep your heart beating, your lungs breathing, your brain thinking, your kidneys filtering, and your body temperature stable while you're lying still in a comfortable room with nothing in your stomach.
BMR represents roughly 60-75% of the total calories you burn each day, making it by far the largest component of your daily energy expenditure. Everything else, movement, digestion, exercise, stress, is layered on top of this baseline.
Understanding your BMR is the foundation of any meaningful conversation about weight management, performance nutrition, or body composition. You simply can't eat for your goals if you don't know what your body needs just to exist.
These three acronyms get used interchangeably on fitness sites. They're related but not identical.
The calories your body burns at complete rest under strict conditions: fasted for 12+ hours, lying down, in a thermally neutral room, immediately after waking. BMR is technically measured in a lab. It's the lowest possible calorie burn your body can hit while alive.
The calories your body burns at rest under more realistic conditions, sitting quietly, not fasted, not first thing in the morning. RMR is typically 10-20% higher than BMR and is what most online calculators (including this one) actually estimate, even when they're labeled "BMR."
The total number of calories you burn in a day, BMR plus everything else: walking to the fridge, typing, digesting food, exercising, fidgeting. TDEE is what you actually need to match if you want to maintain your weight. Use our TDEE calculator to get your number.
In practice: BMR is the floor. TDEE is the ceiling you're trying to match, undershoot (to lose fat), or overshoot (to build muscle). RMR sits in between and is what most calculators return.
Several equations exist to estimate BMR. The gold standard is the Mifflin-St Jeor equation, the formula used by most registered dietitians and validated in the largest body of research. That's what our calculator uses:
Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
All of these formulas are estimates. Individual BMR can vary by up to 15-20% from the equation's prediction because of differences in body composition, genetics, thyroid function, and other factors. The only way to measure BMR precisely is with indirect calorimetry, a device that measures oxygen consumption and CO₂ output.
Your BMR isn't a fixed number. It's shaped by dozens of variables, some you can influence, some you can't.
Larger bodies have more cells, each requiring energy. Heavier people have meaningfully higher BMRs, even at the same body fat percentage.
Muscle is metabolically active tissue. Every pound of muscle burns roughly 6 calories per day at rest, fat burns about 2.
BMR declines roughly 1-2% per decade after age 20, largely due to muscle loss (sarcopenia), not "slower metabolism" in the way the internet says.
Men typically have higher BMRs at the same body size because they carry more muscle and less fat on average.
Thyroid hormones are the master regulators of metabolism. Hypothyroidism can drop BMR by 10-15%; hyperthyroidism can raise it sharply.
Family baseline matters. Some people have naturally "faster" or "slower" metabolisms at identical body compositions, often a 5-10% swing.
Testosterone, estrogen, cortisol, growth hormone, and insulin all shape energy expenditure. Low testosterone in men is linked to measurable drops in BMR.
Chronic under-eating can trigger "metabolic adaptation," where BMR drops 10-15% below what equations predict. This persists even after weight regain.
BMR is the input every other calorie calculation depends on. Whether you're trying to lose fat, gain muscle, or simply maintain, the chain looks like this:
Skip step one and every subsequent number is built on guesswork. Use too high a BMR estimate and you'll think you can eat more than you actually can. Use too low and you'll set an unnecessarily aggressive deficit that triggers fatigue, muscle loss, and rebound weight gain.
Your calorie intake should stay above your BMR in most cases. Dropping below BMR for extended periods can slow thyroid function, disrupt hormones, and trigger metabolic adaptation. A sustainable deficit targets TDEE, not BMR.
You'll eat in a modest surplus over TDEE. Knowing BMR helps you understand how much of your intake is "baseline upkeep" vs. fuel for actual growth.
BMR helps you understand how much food you'd still need even if you suddenly became sedentary (injury, travel, lifestyle change). Lowering calorie intake below BMR on maintenance days is almost always too aggressive.
Marketing loves the phrase "boost your metabolism." Most of those claims are wishful thinking. A handful of strategies actually move BMR, and they're all unglamorous.
The honest answer: muscle mass, hormonal health, and consistent eating patterns do more for BMR than any hack. The fitness industry sells "boost" stories because they sell. Boring beats clever, long term.
For most people, no, not for extended periods. Sustained intake below BMR can cause muscle loss, hormone disruption, loss of menstrual function, thinning hair, and metabolic adaptation (a measurable slowdown in BMR). Very low-calorie diets can be medically appropriate in specific clinical contexts, but only under physician supervision. For general fat loss, stay above BMR and target a deficit below TDEE.
Mifflin-St Jeor, the formula this calculator uses, is accurate within roughly 10% for about 80% of the general population. It's less accurate at extremes of body composition (very muscular or very obese individuals). For precise numbers, indirect calorimetry is the gold standard.
Body composition is the biggest variable. Two 180-pound 35-year-old men can have BMRs that differ by 200+ calories based on muscle mass, genetics, hormone status, and dieting history. Equations treat you as an average person, you probably aren't exactly average.
Yes, but less than people think. A landmark 2021 study (Pontzer et al., Science) found that BMR stays remarkably stable from age 20 to 60 when adjusted for body composition. Most of the "age-related slowdown" people experience is driven by lost muscle, less activity, and changes in body composition, not metabolism itself.
Not precisely. Smart scales and fitness trackers estimate BMR using formulas similar to this calculator, not direct measurement. True BMR testing requires indirect calorimetry, which uses a breath analyzer to measure oxygen consumption. A handful of gyms, clinics, and specialty nutrition practices offer it.
True BMR is measured first thing in the morning, fully rested, in a fasted state. Measuring later in the day or after food gives you RMR, typically 10-20% higher. Online calculators generally estimate something closer to RMR regardless of how they're labeled.
Thyroid hormones (T3 and T4) set the pace of nearly every metabolic process. Hypothyroidism, low thyroid function, can drop BMR by 10-15%, causing weight gain, fatigue, cold intolerance, and low energy. Hyperthyroidism raises BMR and often causes unintentional weight loss. A basic TSH test catches most cases.
Yes. Testosterone supports muscle mass, bone density, and metabolic rate. Men with clinically low testosterone often see measurable drops in BMR and increases in body fat. Restoring normal levels, when medically indicated, can improve body composition and metabolic markers. See our article on signs of low testosterone for more.
BMR is the resting calorie floor. These tools turn that number into something you can act on, total daily burn, deficit math, protein needs, and the hormones underneath your metabolic rate.
Hand-picked guides on metabolism, muscle preservation, and the levers that actually move your resting burn rate.
To lose weight, intake has to fall below maintenance. Crash diets pull you below BMR, that's where muscle loss, hair loss, and rebound start. GLP-1s let you sit comfortably at a moderate deficit because hunger isn't fighting you. In SURMOUNT-1, FDA-approved branded tirzepatide produced an average 20.9% body-weight reduction at 72 weeks; in STEP-1, FDA-approved branded semaglutide produced 14.9% at 68 weeks. Trial participants achieved these outcomes at moderate sustainable deficits — not crash dieting below BMR. Compounded preparations are not FDA-approved.
How this tool calculates
Basal metabolic rate is estimated using the Mifflin-St Jeor equation (1990), which is the most accurate predictive equation for resting energy expenditure in healthy adults per systematic review of validation studies.
Peer-reviewed sources
Important. This tool is provided for educational purposes only and does not constitute medical advice, diagnosis, or treatment. The tool does not prescribe medication, recommend specific dosing, or substitute for clinical evaluation. Compounded medications referenced anywhere on this site are not FDA-approved; the FDA does not verify the safety, effectiveness, or quality of compounded drugs. Treatment decisions are made only by a licensed U.S. physician after individual patient evaluation.