Your basal metabolic rate is what your body costs to run when you’re doing nothing at all. Lying still, awake, fasted, in a comfortable room — heart beating, lungs working, kidneys filtering, brain consuming its considerable share.
For most people it’s 60–70% of everything they burn in a day, which surprises anyone who assumes exercise dominates. It doesn’t. You spend more calories being alive than working out.
BMR Calculator
The calories your body burns doing nothing at all — breathing, pumping blood, staying warm.
Methods: Mifflin–St Jeor is the headline number and the most accurate general-population equation. Revised Harris–Benedict and Katch–McArdle (lean-mass based, needs a body fat entry) are shown for comparison. Individual metabolisms vary roughly ±10% around any of them.
Estimates only — general education, not medical advice. Talk with your doctor before changing how you eat or train. More tools at whatshouldiweigh.com.
What’s actually consuming it
Your organs do most of the work, and not in proportion to their size.
The liver and brain are the biggest single consumers, each accounting for roughly a fifth of resting energy use. Your brain alone runs on about 20% of your total energy while making up 2% of your bodyweight. Heart and kidneys are small but metabolically expensive. Skeletal muscle, despite being the largest tissue in the body, uses a modest amount at rest — around 20% of the total, spread across a great deal of mass.
Fat tissue is the least demanding, at roughly 2 calories per pound per day against muscle’s 6.
That breakdown explains something people find counterintuitive: larger people have higher BMRs, including larger people carrying more fat. More body means more tissue to maintain, more blood to circulate, bigger organs. Someone at 250 pounds has a substantially higher resting burn than someone at 150.
The three equations
The calculator runs all three.
Mifflin-St Jeor (1990) is the headline figure and the current default recommendation for healthy adults. It was built on more representative data than its predecessors and predicts measured resting metabolism within 10% for most people.
Harris-Benedict (1919, revised 1984) is the original. It was derived from a small, relatively lean early-twentieth-century sample and tends to overestimate modern populations by around 5%. Still widely cited, largely from habit.
Katch-McArdle works from lean body mass instead of total weight, which sidesteps the whole muscle-versus-fat problem. If you know your body fat percentage it’s often the most accurate of the three — particularly at the extremes, for very lean or very muscular people, where the weight-based formulas struggle.
If you have a body fat figure, enter it. If not, Mifflin is the sensible default.
Why yours differs from someone your size
Two people matching on age, sex, height and weight can differ by 200–300 calories a day in measured resting metabolism. Any formula gives them the same answer.
The sources of that variation:
Body composition. The big one, and the reason weight-based formulas are imprecise. More lean mass at the same weight means a higher BMR.
Organ size relative to body size. Genuinely variable between individuals, and not something you can influence.
Thyroid function. Thyroid hormones set the pace of cellular metabolism. Hypothyroidism can lower BMR meaningfully — it’s one of the few medical explanations for unexplained weight change that’s worth ruling out with a blood test.
Genetics. A real and measurable contribution, though smaller than people hoping for an explanation would like.
Recent dieting history. Extended deficits suppress resting metabolism somewhat beyond what tissue loss predicts.
Can you raise it?
Mostly, not by much — and the honest answer is worth more than the optimistic one.
Build muscle. Real, but modest. Ten pounds of added muscle raises resting burn by roughly 40 calories a day. Worth doing for strength, function, appearance and insulin sensitivity; a weak strategy if the goal is metabolic rate.
Eat enough. Prolonged severe restriction lowers BMR. Not restricting severely avoids that. This is prevention rather than enhancement.
Treat an underlying condition. If thyroid function is genuinely impaired, addressing it restores what was lost. That’s a doctor’s job, not a supplement’s.
Things that don’t work in any meaningful way: eating small frequent meals, green tea, cold showers, spicy food, metabolism-boosting supplements. Several produce measurable effects in laboratory conditions that amount to a handful of calories a day in real life.
“Metabolic damage” — what’s true and what isn’t
You’ll see claims that dieting permanently wrecks your metabolism. The reality is more moderate.
Adaptive thermogenesis is real: after sustained weight loss, resting metabolism sits somewhat lower than body composition alone would predict. Studies of substantial weight loss have measured suppression in the range of 100–300 calories a day, and follow-up work on extreme cases has found it persisting for years.
But two things get lost in the retelling. The effect is usually a few hundred calories, not the thousand-plus sometimes implied. And it’s largely proportional to how aggressive the deficit was — slower loss with adequate protein and resistance training produces considerably less of it.
The practical takeaway isn’t to fear dieting. It’s an argument for moderate deficits over crash approaches, and for keeping muscle while you lose fat.
Your BMR is not a calorie target
Worth stating plainly, because people get this wrong in a way that hurts them.
BMR is what you’d burn in a coma. It doesn’t include standing up, walking to the kitchen, working, or training. Eating at your BMR means eating well under what you actually spend — an aggressive deficit, whether or not you intended one.
To get a figure you can eat to, multiply BMR by an activity factor. That’s what the [daily calorie calculator] does, and it’s the number worth using.
Common questions
What is a normal BMR? Roughly 1,600–1,800 calories for an average adult man and 1,300–1,500 for an average adult woman, but it scales with body size and the range is wide. Use the calculator rather than a typical figure.
Is BMR the same as RMR? Nearly. Basal metabolic rate is measured under strict laboratory conditions — fasted, fully rested, temperature-controlled. Resting metabolic rate is measured more loosely and comes out around 10% higher. The terms are used interchangeably outside research.
Why is my BMR lower than my friend’s? Body size is the dominant factor, then lean mass, then age and sex. A smaller body with less muscle costs less to run.
Does BMR fall with age? Yes, though slower than commonly assumed. Recent large-scale research found total energy expenditure is remarkably stable from about 20 to 60 before declining. Much of what gets blamed on age is actually muscle loss and reduced activity, both of which are partly addressable.
Can I measure my actual BMR? Yes — indirect calorimetry, offered by some sports labs and clinics, measures oxygen consumption directly. It’s the only way to know rather than estimate, though for most purposes tracking your weight against a calculated figure for a few weeks gets you close enough.
The calculators on this site are educational estimates, not medical advice. If you suspect a thyroid or metabolic condition, that’s a conversation for your doctor and a blood test, not a calculator.