What basal metabolic rate is
Basal metabolic rate is the energy your body needs at complete rest to run its essential processes: circulation, breathing, cell repair, brain activity, and maintaining body temperature. It accounts for 60% to 70% of most people's total daily calorie use, which makes it the largest single component by a wide margin.
Strictly measured, BMR requires a fasted subject at complete rest in a thermally neutral room. Resting metabolic rate (RMR) is the practical approximation taken under less strict conditions and runs slightly higher. In everyday use the two terms are treated as interchangeable.
| Component | Share of daily expenditure | What it covers |
|---|---|---|
| Basal metabolic rate | 60 – 70% | Breathing, circulation, temperature, cell repair, brain activity |
| Thermic effect of food | about 10% | Digesting and absorbing what you eat; highest for protein |
| Physical activity | 15 – 30% | Deliberate exercise plus everyday movement (NEAT) |
What determines your BMR
Five factors dominate, and only some are within your control.
- Lean body mass: the strongest driver. Muscle, organs, and other metabolically active tissue burn energy continuously, which is why two people of the same weight can have different BMRs.
- Body size: larger bodies need more energy simply to maintain themselves.
- Age: BMR declines gradually through adulthood, driven substantially by muscle loss rather than age itself.
- Sex: men typically have higher BMRs at the same height and weight, mostly explained by greater average lean mass.
- Genetics, thyroid function, and hormones: these produce real individual variation of 10% or more in either direction from any equation's prediction.
| Factor | Direction | Typical size of effect |
|---|---|---|
| More lean body mass | Raises | Roughly 6 kcal a day at rest per pound of muscle — real, but far less than often claimed |
| Ageing | Lowers | About 1 – 2% per decade after early adulthood, mostly through lost muscle |
| Fever or raised body temperature | Raises | Roughly 7% for each 1 °F (0.55 °C) above normal |
| Overactive thyroid | Raises | Can be substantial; a common medical cause of an unexpectedly high BMR |
| Underactive thyroid | Lowers | Can be substantial; worth testing if BMR seems far below prediction |
| Prolonged severe calorie restriction | Lowers | Around 10 – 15% beyond what the weight loss alone predicts (adaptive thermogenesis) |
| Caffeine and other stimulants | Raises | A few percent, for a few hours |
| Cold exposure | Raises | Modest and short-lived, through shivering and brown-fat activity |
| Pregnancy and breastfeeding | Raises | Increasingly through pregnancy; several hundred kcal a day while breastfeeding |
Which equation to trust
The Mifflin-St Jeor equation, published in 1990, is the current standard and the one shown first above. A systematic review by the American Dietetic Association found it predicted resting energy expenditure within 10% of measured values more often than the alternatives.
The revised Harris-Benedict equation, originally from 1919 and updated in 1984, is also shown for comparison and usually reads slightly higher. If you have substantially more or less muscle than average and know your body fat percentage, the Katch-McArdle formula — which uses lean mass directly — can outperform both; enter a body fat figure above to see it.
| Equation | Men | Women | Best suited to |
|---|---|---|---|
| Mifflin-St Jeor (1990) | 10W + 6.25H − 5A + 5 | 10W + 6.25H − 5A − 161 | Most adults; the recommended default |
| Revised Harris-Benedict (1984) | 88.362 + 13.397W + 4.799H − 5.677A | 447.593 + 9.247W + 3.098H − 4.330A | Comparison with older references |
| Katch-McArdle | 370 + 21.6 × lean mass | 370 + 21.6 × lean mass | Lean or muscular people with a known body fat % |
From BMR to what you should eat
BMR on its own is not a target for anything. Total daily energy expenditure — what you actually burn — is BMR multiplied by an activity factor, and that is the figure to eat at to maintain weight or to eat moderately below to lose it. The calculator lists your daily need at every level; pick the row that describes a typical week, and when in doubt take the lower one.
| Level | Typical week | Multiplier |
|---|---|---|
| Sedentary | Desk work, little or no deliberate exercise | 1.20 |
| Light | Exercise 1–3 times a week | 1.375 |
| Moderate | Exercise 4–5 times a week | 1.465 |
| Active | Daily exercise, or intense exercise 3–4 times a week | 1.55 |
| Very active | Intense exercise 6–7 times a week | 1.725 |
| Extra active | Very intense exercise daily, or a physically demanding job | 1.90 |
Can you raise your BMR?
Modestly, and mostly through building muscle. Resistance training adds metabolically active tissue, though the direct effect is often overstated: a pound of muscle burns roughly 6 calories a day at rest, not the 50 sometimes claimed. The larger benefits of training are improved insulin sensitivity, better body composition, and the calories burned during and after the sessions themselves.
Severe or prolonged calorie restriction moves BMR the other way, a phenomenon called adaptive thermogenesis. Losing weight slowly while eating adequate protein and training with resistance preserves more lean mass and limits that decline.