What is BMR (basal metabolic rate)?
Your basal metabolic rate is what your body spends just staying alive. Lie perfectly still in a dark room for 24 hours and you'd still burn a surprising amount of energy — your heart beats about a hundred thousand times, your liver processes everything in your blood, your brain alone claims roughly a fifth of the total. That baseline cost is your BMR, and for most people it's the single biggest part of everything they burn in a day.
Knowing it matters because every calorie plan is built on top of it. Get the foundation wrong and the targets stacked on it — maintenance, deficit, surplus — inherit the error.
The three BMR formulas this calculator runs
Nobody can measure your true BMR outside a lab, so researchers have built predictive formulas from thousands of measured subjects. This calculator runs three of the best-known ones side by side, so you can see how much (or little) they disagree for your body:
Mifflin-St Jeor (the headline number)
Men: 10 × kg + 6.25 × cm − 5 × age + 5
Women: 10 × kg + 6.25 × cm − 5 × age − 161
Published in 1990 and validated more times than any rival. When studies pit the formulas against lab measurements, this one wins most often, which is why it drives your main result.
Harris-Benedict (the classic)
The original BMR equation dates back to 1918, revised in 1984. It tends to read a little high for modern, more sedentary bodies, but it's still widely quoted, so the calculator shows it for comparison.
Katch-McArdle (for lean people who know their body fat)
370 + 21.6 × lean mass (kg)
This one ignores age and sex entirely and works from lean body mass, which is what actually burns the energy. It shines for muscular people whom the other formulas underestimate. You can get your lean mass figure from the lean body mass calculator.
A worked example: a 30-year-old woman, 65 kg and 165 cm, gets a Mifflin-St Jeor BMR of 650 + 1,031 − 150 − 161, which is about 1,370 calories a day. Before any walking, working, or workouts.
BMR vs TDEE: which one should you use?
A mistake people make constantly: eating at their BMR to lose weight. Your BMR is the floor, not the target. The moment you get out of bed you're burning more, and your real daily burn — TDEE — is typically 1.4 to 1.9 times your BMR depending on how active you are.
This page shows both numbers, but if daily burn is what you're really after, the TDEE calculator is built around it, and the calorie calculator turns it into goal-specific targets.
What raises or lowers your BMR
Two people the same height and weight can have noticeably different metabolic rates, and the reasons are mostly unglamorous. Muscle is the big one — it burns several times more energy at rest than fat does, which is why lifting weights pays a quiet dividend around the clock. Age works against you at roughly one to two percent per decade after your twenties, largely because muscle slips away. Thyroid hormones set the overall tempo. And prolonged dieting slows things down as your body adapts to scarcity — part of why the last few pounds always fight back.
None of these are captured perfectly by any formula, which is why your calculated BMR is a well-informed estimate rather than a lab result. Treat it as the starting line.
Frequently asked questions about BMR
What is BMR and why does it matter?
BMR (basal metabolic rate) is the calories your body burns at complete rest to keep organs functioning. It is the baseline for estimating daily calorie needs before activity is added.
Which BMR formula is most accurate?
Mifflin-St Jeor is the most widely used for general estimates. Katch-McArdle can be better if you know your body fat percentage. All formulas are population averages — use results as a starting point.
What is the difference between BMR and TDEE?
BMR is calories at rest only. TDEE (total daily energy expenditure) adds activity — walking, exercise, and daily movement. Weight-loss plans usually target TDEE, not BMR alone.
How should I use BMR to lose or gain weight?
To lose weight, eat below your TDEE; to gain, eat above it. A common starting deficit is 300–500 calories below TDEE. Track weight for 2–3 weeks and adjust if progress stalls.
When should I talk to a doctor or dietitian about calories?
Seek professional guidance if you have diabetes, kidney disease, a history of eating disorders, are pregnant, or take medications that affect metabolism.
How do I use this BMR calculator?
Enter gender, age, height, weight, and activity level, then click Calculate BMR. Your BMR, TDEE, and calorie targets appear on the results panel.
Sources and further reading
- Mifflin MD et al. — A new predictive equation for resting energy expenditure (Am J Clin Nutr, 1990)
- Frankenfield D et al. — Comparison of predictive equations for resting metabolic rate (J Am Diet Assoc, 2005)
- Harris JA, Benedict FG — A biometric study of human basal metabolism (PNAS, 1918)
- MedlinePlus — Metabolism and weight
This page is for general information only and is not medical or dietary advice. Talk to a qualified professional before making major changes to how you eat.