Four formulas at once, because reporting one to the calorie implies a precision none of them have. The spread between them is the honest margin, and the activity multiplier below is where the real uncertainty sits.
BMR Calculator
LiveWhat BMR is, and what it is not
Basal metabolic rate is the part of your metabolism that runs whatever you do — the energy your body uses doing nothing at all — breathing, circulating blood, maintaining temperature, repairing tissue, running the brain. It is measured in a lab after an overnight fast, lying still, at a controlled temperature.
Nothing on this page measures that. These are equations fitted to populations, and they describe the average person of your sex, age, height and weight. An individual can sit 10% either side of the prediction for reasons no equation captures — thyroid function, past dieting history, muscle distribution, genetics.
| Term | Includes | Roughly |
|---|---|---|
| BMR | Basal functions only | 60–70% of daily burn for most people |
| RMR | The same, measured less strictly | Usually 3–10% above BMR. The terms are often used interchangeably |
| TEF | Digesting food | About 10% of what you eat, higher on protein |
| NEAT | Fidgeting, standing, walking about | Varies by up to 2,000 kcal between individuals |
| Exercise | Deliberate training | Usually the smallest component, despite the attention it gets |
| TDEE | All of the above | What the activity multiplier is trying to estimate |
The four formulas
| Formula | Year | Needs | Best for |
|---|---|---|---|
| Mifflin-St Jeor | 1990 | Sex, age, height, weight | The default. The most accurate for the general population in validation studies, and what most clinicians use |
| Harris-Benedict Roza revision | 1919, revised 1984 | The same | The historical standard. Tends to read a little high, particularly in people carrying more fat |
| Katch-McArdle | 1996 | Body fat percentage | The best option if you know your body fat, because it works from lean mass rather than total weight |
| Cunningham | 1980 | Body fat percentage | Reads highest. Fitted on athletes, so it suits lean, trained people and overestimates for everyone else |
Men: 10 × kg + 6.25 × cm − 5 × age + 5
Women: 10 × kg + 6.25 × cm − 5 × age − 161
The two body-composition formulas ignore sex entirely, which sounds like an oversight and is not: the reason men and women differ in the other equations is mostly that they differ in lean mass, and once lean mass is measured directly there is nothing left for a sex term to explain.
Why the activity multiplier is the real problem
The formulas disagree by around 12%. The activity dropdown disagrees with itself by 58%:
| Level | Factor | What it honestly describes |
|---|---|---|
| Sedentary | ×1.2 | Desk job, a car everywhere, no deliberate exercise. Fewer than about 5,000 steps a day |
| Light | ×1.375 | Mostly seated, plus one to three sessions a week. Or a job on your feet with no training |
| Moderate | ×1.55 | Three to five real sessions a week, and generally on the move otherwise |
| Active | ×1.725 | Hard training six or seven days a week. Not three gym visits and a walk |
| Very active | ×1.9 | Manual labour, or twice-daily training. Very few people belong here |
If the number matters, the multiplier is not the way to get it. Track what you actually eat for two or three weeks and watch your weight; if it holds steady, that intake is your maintenance, measured rather than predicted. Every equation on this page is a starting point for that experiment, not a substitute for it.
From BMR to maintenance calories
Basal metabolic rate is not a number you can eat. The figure people actually want is maintenance calories — the intake at which weight holds steady — and BMR is one component of it:
BMR × activity factor = TDEE
TDEE = maintenance calories
maintenance − deficit = loss
maintenance + surplus = gain
| Goal | Typical adjustment | What it means in practice |
|---|---|---|
| Maintain | TDEE | Weight steady over a fortnight. This is what your resting metabolic rate and daily movement together demand |
| Slow loss | −10 to 15% | Roughly 0.25 to 0.5 kg a week. Preserves muscle best and is the easiest to sustain |
| Faster loss | −20 to 25% | Around 0.5 to 0.75 kg a week. Harder to hold, and more muscle goes with it |
| Slow gain | +5 to 10% | Adds tissue with less fat than a large surplus does |
How many calories anyone needs is genuinely individual, and no equation settles it. If a goal matters, use the figure above as a starting point, hold it for two or three weeks, and let the scale tell you whether the estimate was right for you. Persistent fatigue, hair loss, cold hands, a stalled cycle or an eating pattern that feels compulsive are reasons to stop and speak to a doctor or a registered dietitian, not to subtract more.
Why the number drifts over time
| Cause | Effect |
|---|---|
| Losing weight | A smaller body costs less to run. Losing 10 kg drops BMR by roughly 100 kcal before anything else happens |
| Adaptive thermogenesis | Prolonged restriction lowers metabolic rate beyond what the weight change predicts — commonly cited at 10 to 15% below prediction |
| Losing muscle | Lean tissue is the metabolically active part. Dieting without resistance training loses some of it, and the rate follows |
| Ageing | Metabolism slows around 1 to 2% a decade after 20, much of it explained by lost muscle rather than age itself |
| Thyroid function | Can move BMR substantially in either direction. Outside what any equation models |
Sources
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. “A new predictive equation for resting energy expenditure in healthy individuals.” American Journal of Clinical Nutrition, 1990;51(2):241–247. The 498-subject study that produced the equation now used as the clinical default.
- Harris JA, Benedict FG. A Biometric Study of Basal Metabolism in Man. Carnegie Institution of Washington, 1919. The original measurements, on 239 subjects.
- Roza AM, Shizgal HM. “The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass.” American Journal of Clinical Nutrition, 1984;40(1):168–182. The revision this page uses, correcting arithmetic in the 1919 coefficients.
- Cunningham JJ. “A reanalysis of the factors influencing basal metabolic rate in normal adults.” American Journal of Clinical Nutrition, 1980;33(11):2372–2374. The lean-mass equation fitted on an athletic sample.
- Katch FI, McArdle WD. Exercise Physiology: Energy, Nutrition and Human Performance. The lean-mass equation in general use.
- Frankenfield D, Roth-Yousey L, Compher C. “Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review.” Journal of the American Dietetic Association, 2005;105(5):775–789. The review that established Mifflin-St Jeor as the most accurate of the group.
- Elia M. “Organ and tissue contribution to metabolic rate.” In Energy Metabolism: Tissue Determinants and Cellular Corollaries, Raven Press, 1992. The per-kilogram figures for liver, brain, muscle and fat quoted above.
- Rosenbaum M, Leibel RL. “Adaptive thermogenesis in humans.” International Journal of Obesity, 2010;34(S1):S47–S55. The evidence behind the 10 to 15% drop below prediction after sustained weight loss.
- Levine JA. “Non-exercise activity thermogenesis (NEAT).” Best Practice & Research Clinical Endocrinology & Metabolism, 2002;16(4):679–702. The source for the very wide between-person range in incidental movement.
Every figure on this page is computed from the equations as published in these papers. The activity multipliers are the exception — they are the conventional Harris-Benedict factors, in general use for decades and without a comparable evidence base. That is why this page treats the multiplier as the largest source of uncertainty rather than a settled input.
How the equations were built
Each of these formulas is a regression fitted to people who were actually measured in a lab. Knowing who those people were explains most of the disagreement between them:
| Formula | Sample | Reference method | What it inherits |
|---|---|---|---|
| Harris-Benedict 1919 | 136 men, 103 women, mostly lean and young | Indirect calorimetry, early apparatus | A sample nothing like a modern population. Reads high, particularly in people carrying more fat |
| Roza revision 1984 | The original data, recomputed | The same | Corrected arithmetic in the 1919 coefficients. Still the 1919 sample |
| Mifflin-St Jeor 1990 | 498 adults, both sexes, wide weight range | Indirect calorimetry | The most representative sample of the four. Why it became the clinical default |
| Katch-McArdle 1996 | Fitted on lean mass rather than weight | Body composition analysis | Accuracy that depends entirely on the body fat figure you feed it |
| Cunningham 1980 | 223 subjects, athletic | Body composition analysis | Reads highest. Suits trained people and overestimates everyone else |
None of them was fitted on you. A regression describes the average of its sample and nothing about how far an individual sits from it — validation work on Mifflin-St Jeor typically finds it within 10% for around 80% of people, which also means one person in five is further out than that.
What actually moves your metabolic rate
The equations use weight, height, age and sex because those are easy to measure, not because they are the mechanism. What the mechanism actually consists of:
| Tissue or factor | Rate | Note |
|---|---|---|
| Liver | ~200 kcal/kg/day | About 2% of body weight and a fifth of resting energy use |
| Brain | ~240 kcal/kg/day | Roughly 20% of the total, and almost fixed regardless of body size |
| Heart and kidneys | ~440 kcal/kg/day | Small organs, disproportionate cost |
| Skeletal muscle | ~13 kcal/kg/day | Far lower than commonly claimed. A kilogram of muscle adds about 13 kcal a day, not 50 |
| Fat tissue | ~4.5 kcal/kg/day | Not inert, but close to it |
Common mistakes
Frequently asked questions
Which BMR formula is most accurate?
Mifflin-St Jeor for most people — it performed best in validation work and is what clinicians generally use. If you have a reliable body fat measurement, Katch-McArdle is better still, because it works from lean mass. Cunningham reads highest and was fitted on athletes, so it suits lean trained people and overestimates for everyone else.
Why do the formulas give different answers?
Because each was fitted to a different group of people at a different time. Harris-Benedict came from 1919 data on a small sample; Mifflin-St Jeor from a larger and more modern one; the body-composition formulas from lean mass rather than total weight. The spread between them is the honest uncertainty, which is why all four are shown.
What activity level should I choose?
Almost certainly one lower than you think. Three gym sessions a week with a desk job is light, not moderate, and an hour of exercise does not offset eight hours of sitting. This choice moves the result more than the formula does — the gap between sedentary and very active is over 1,200 kcal for the same person.
What is the difference between BMR and TDEE?
BMR is what a body at complete rest uses — typically 60 to 70% of the daily total. TDEE adds digestion, incidental movement and exercise on top. TDEE is the figure that means anything for planning; BMR is the component it is built from.
Does BMR fall when you lose weight?
Yes, for two reasons. A smaller body costs less to run — about 100 kcal less per 10 kg. On top of that, prolonged restriction lowers the rate further than the weight change alone predicts, an effect commonly reported at 10 to 15%. Keeping resistance training in the plan preserves lean mass and limits the first part.
How many calories do I need a day?
Your maintenance calories are BMR multiplied by the activity factor — your daily calorie burn shown above. For a goal, adjust by a percentage of that rather than a fixed number: 10 to 15% below for slow loss, 5 to 10% above for slow gain. A 500-calorie deficit is a tenth of the day for one person and a third for another, which is why percentages travel better.
What is the difference between BMR and RMR?
Resting metabolic rate is measured under less strict conditions than basal metabolic rate — no overnight fast in a lab, no controlled temperature — and comes out 3 to 10% higher as a result. In everyday use the terms are treated as interchangeable, and the equations on this page are usually described as predicting either.
How accurate is this?
Around 10% for an individual, at best, before the activity multiplier adds its own uncertainty. If the number genuinely matters, track your intake for two or three weeks and watch your weight: if it holds steady, that intake is your maintenance, measured rather than predicted. The equations are a starting point for that, not a replacement.
Should I know my body fat percentage?
Only if the measurement is reliable. Katch-McArdle and Cunningham are better than the others when the input is real, and worse when it is guessed — a bathroom scale can be eight points out, which moves the answer more than switching formulas would. DEXA and hydrostatic weighing are dependable; consumer scales are not.
Is my data stored?
No. Everything runs in your browser with no server request, and works offline once the page has loaded. Nothing is written to disk and nothing persists after you close the tab.
Related calculators
See the full list of Health calculators, or try:
- Calorie Calculator — intake for a goal, with clinical floors
- TDEE Calculator — the full daily figure
- Body Fat Calculator — the input the lean-mass formulas need
- BMI Calculator — eight scales and their limits
- Macro Calculator — splitting the total between protein, fat and carbohydrate
- Ideal Weight Calculator — four competing definitions