BMR, RMR, TDEE — three different numbers
BMR (basal metabolic rate) is the energy cost of being alive and doing nothing: heart beating, lungs moving, kidneys filtering, brain running, body held at temperature. Strictly measured, it requires a 12-hour fast, a full night's rest, a thermally neutral room, and a subject lying awake and still. It typically accounts for 60–70% of what you burn in a day.
RMR (resting metabolic rate) is the same measurement taken under realistic conditions — you drove to the lab, you had coffee four hours ago. RMR runs about 5–10% higher than true BMR. Every prediction equation on this page was fitted to lab data of one kind or the other, so the distinction is mostly academic for a calculator; use whichever word your source uses.
TDEE (total daily energy expenditure) is BMR plus digestion, plus daily movement, plus training. It is the number you set calories against, and it is 1.2 to 1.9 times your BMR depending on how much you move. If you came here to work out what to eat, you want the TDEE calculator; BMR is the input it starts from.
The three equations
Mifflin-St Jeor (1990) — the default
Repeatedly validated as the most accurate general equation for healthy adults, which is why the Academy of Nutrition and Dietetics recommends it. Use it unless you have a real body-fat measurement.
Revised Harris-Benedict (1984)
The 1919 original, refitted by Roza and Shizgal. It reads a little high for modern populations — commonly 5% or so above Mifflin — and is included mainly because a great deal of older diet literature is built on it.
Katch-McArdle (1996)
No sex term, no age term — just lean mass, since lean mass is what actually consumes the energy. That makes it the best of the three for very muscular or very lean people, whom the height-and-weight equations systematically underestimate. It needs a body fat percentage; get one from a scan or estimate it with the body fat calculator, and see the lean body mass calculator for the input on its own.
Where your BMR actually goes
The donut above splits resting energy by tissue, and the split is the most counter-intuitive fact in metabolism. The brain, liver, heart and kidneys together are under 6% of your body mass and spend more than half of your resting energy. Skeletal muscle is 40% of your mass and spends about a fifth.
| Tissue | Metabolic rate | Share of RMR (men) |
|---|---|---|
| Heart, kidneys | 440 kcal/kg/day | ~17% |
| Brain | 240 kcal/kg/day | ~19% |
| Liver | 200 kcal/kg/day | ~17% |
| Skeletal muscle | 13 kcal/kg/day | ~24% |
| Fat tissue | 4.5 kcal/kg/day | ~4% |
The practical consequence is a deflating one: a kilogram of new muscle adds around 13 kcal/day to resting burn, not the 50 or 100 that gets quoted. Muscle is worth building for a dozen other reasons — insulin sensitivity, strength, injury resistance, appearance — but its direct contribution to resting metabolism is modest. The reason a muscular person has a high BMR is that they carry more of every metabolic tissue, not that muscle itself is a furnace.
What actually moves BMR
- Body size. The dominant term by far. More mass of any kind means more BMR; more lean mass means proportionally more.
- Lean mass. The part you control. Resistance training is the only durable lever.
- Energy deficit. Sustained under-eating lowers resting metabolism beyond what the weight loss predicts, typically by something like 5–15% at large deficits. This is real adaptive thermogenesis, and it is reversible.
- Thyroid function. Genuinely large effects, in both directions. Worth investigating if your measured numbers are far off prediction.
- Nothing in a supplement. Green tea, capsaicin and caffeine produce measurable but trivial changes — tens of calories, for hours, with tolerance.
On “starvation mode”
The claim that eating too little stops weight loss because your metabolism “shuts down” is false at the scale it is usually asserted. Adaptive thermogenesis is a discount, not a wall: the Minnesota Starvation Experiment's subjects, on roughly half their maintenance intake for 24 weeks, lost about a quarter of their body weight. What actually stalls diets is a shrinking body needing fewer calories, combined with quietly reduced daily movement and gradually looser adherence. All three are fixable, and none of them are your metabolism breaking.
Measuring it properly
The reference method is indirect calorimetry: you breathe into a hood or mask for 10–30 minutes while the machine measures oxygen consumed and carbon dioxide produced, and energy expenditure is computed from the gas exchange. Done well, it is accurate to within a few percent — a different class of number from anything a formula produces. Sports-medicine clinics and larger hospital nutrition departments offer it, usually for a modest fee.
If you are not going to measure it, the practical alternative is calibration: take the calculated figure, add activity to get a TDEE, eat at that number for two to three weeks, and compare your average weekly weight against the prediction. Two weeks of honest logging beats any equation on this page.