What Is Basal Metabolic Rate?
Basal metabolic rate (BMR) is the energy your body uses at complete rest to keep itself alive: circulating blood, breathing, maintaining body temperature, growing and repairing cells, and running the brain and nervous system. It is measured in a strictly controlled state — lying still, fully rested, in a neutral temperature, at least twelve hours after eating.
BMR is the largest single component of daily energy expenditure, typically 60–70% of the total for a sedentary person. Whatever you do with the rest of your day, this is the floor.
BMR, RMR and TDEE
| Term | What it measures | Typical relationship |
|---|---|---|
| BMR | Energy at complete rest under laboratory conditions | Baseline |
| RMR | Resting metabolic rate, measured under less strict conditions | About 5–10% higher than BMR |
| TDEE | Total daily energy expenditure, including all activity and digestion | BMR × 1.2 to 1.9 |
In everyday use BMR and RMR are treated interchangeably, and the equations below are commonly labelled either way. TDEE is what you actually eat against.
The Three Equations
Mifflin-St Jeor (1990)
Developed on a modern population and validated repeatedly since. It is the equation most dietitians use, and the default here.
Men: BMR = 10W + 6.25H − 5A + 5
Women: BMR = 10W + 6.25H − 5A − 161
Revised Harris-Benedict (1984)
The original 1919 equation was built on a small sample of lean, young subjects and overestimates BMR for most modern populations by around 5%. The 1984 revision corrected much of that, but Mifflin-St Jeor is still more accurate on average.
Men: BMR = 13.397W + 4.799H − 5.677A + 88.362
Women: BMR = 9.247W + 3.098H − 4.330A + 447.593
Katch-McArdle
The only one of the three that uses body composition rather than total weight. Because muscle is metabolically active and fat is much less so, two people of identical weight can have quite different BMRs. If you have a reliable body fat measurement — DEXA, hydrostatic weighing, or a good caliper reading — this is the most accurate equation for you. If you are guessing at the number, it is the least.
BMR = 370 + 21.6 × LBM where LBM = W × (1 − body fat fraction)
In all equations W is weight in kilograms, H is height in centimetres and A is age in years.
What Determines Your BMR
- Body size — more tissue requires more energy. This is the largest factor.
- Lean mass — muscle burns roughly 13 Calories per kilogram per day at rest, fat around 4.5. Two people at the same weight differ by a few hundred Calories if their body composition differs sharply.
- Age — BMR falls with age, mainly because muscle mass declines. The drop is roughly 1–2% per decade after 20, and resistance training slows it substantially.
- Sex — men average higher BMRs than women of the same height and weight, largely because of greater lean mass.
- Genetics — individuals vary by up to 10–15% around the predicted value for reasons not captured by any equation.
- Thyroid function — hyperthyroidism raises BMR markedly; hypothyroidism lowers it.
- Body temperature — each degree Celsius of fever raises BMR by roughly 10–13%.
- Diet history — prolonged severe restriction lowers BMR beyond what weight loss alone predicts, an effect called adaptive thermogenesis.
The Components of Daily Energy Use
| Component | Share of total | Notes |
|---|---|---|
| Basal metabolic rate | 60–70% | Largely fixed in the short term |
| Thermic effect of food | ~10% | Highest for protein, lowest for fat |
| Exercise activity | 5–15% | Deliberate training |
| Non-exercise activity (NEAT) | 5–25% | Fidgeting, walking, standing — varies enormously between individuals |
NEAT is the most variable component and the most overlooked. Differences in unconscious daily movement can account for several hundred Calories a day between two otherwise similar people.
Can You Raise Your BMR?
Within limits, and less than the marketing around it suggests.
- Build muscle. The most reliable lever, though the effect is modest: each kilogram of added muscle raises BMR by roughly 13 Calories a day. The larger benefit is the training itself and improved insulin sensitivity.
- Avoid crash dieting. Severe restriction reduces BMR by more than the weight loss alone would predict, and the effect persists after weight regain.
- Eat enough protein. Protein has the highest thermic effect at 20–30% of its calories, versus 5–10% for carbohydrate and 0–3% for fat. This raises TDEE rather than BMR, but the result is the same.
- Sleep adequately. Chronic short sleep is associated with reduced energy expenditure and disrupted appetite regulation.
Caffeine and capsaicin produce measurable but small and temporary increases. "Metabolism-boosting" supplements generally do not produce clinically meaningful effects.
Frequently Asked Questions
Which equation should I use?
Mifflin-St Jeor unless you have a measured body fat percentage, in which case Katch-McArdle. Harris-Benedict is included for comparison and because some references still use it.
How accurate is a BMR estimate?
The equations predict within about 10% for most people, but individual variation is real. Use the result as a starting point and adjust based on how your weight actually responds over three to four weeks.
Should I eat below my BMR to lose weight?
Generally no. Eating below BMR for extended periods makes adequate nutrition difficult and accelerates muscle loss. Create the deficit against TDEE instead, which is considerably higher.
Why did my BMR fall after losing weight?
Partly because there is less body to maintain, and partly through metabolic adaptation. Recalculate at your new weight rather than continuing with the old target.