Your basal metabolic rate is the energy your body spends staying alive at complete rest — running your heart, brain, kidneys, and the constant turnover of tissue. It is the floor beneath every other number in nutrition, typically 60–70% of everything you burn in a day.
- What it measures
- Energy used at complete rest
- Default equation
- Mifflin-St Jeor (1990)
- Share of daily burn
- Typically 60–70%
- Needs
- Sex, age, height, weight
- Typical accuracy
- Within 10% for most adults
Formula
Mifflin-St Jeor (1990)Men: BMR = (10 × weight) + (6.25 × height) − (5 × age) + 5 Women: BMR = (10 × weight) + (6.25 × height) − (5 × age) − 161
- weight
- — Body weight in kilograms
- height
- — Height in centimetres
- age
- — Age in years
Mifflin-St Jeor is the default because the Academy of Nutrition and Dietetics found it predicts measured resting energy expenditure within 10% more reliably than Harris-Benedict, in both normal-weight and obese adults.
BMR Calculator examples
| Input | Result | Category |
|---|---|---|
| Man, 30, 5'10", 180 lb | 1,783 cal/day | Basal metabolic rate |
| Man, 45, 6'0", 200 lb | 1,830 cal/day | Basal metabolic rate |
| Man, 25, 5'8", 160 lb | 1,685 cal/day | Basal metabolic rate |
| Woman, 30, 5'5", 150 lb | 1,401 cal/day | Basal metabolic rate |
| Woman, 45, 5'4", 140 lb | 1,265 cal/day | Basal metabolic rate |
| Woman, 25, 5'7", 130 lb | 1,367 cal/day | Basal metabolic rate |
What is basal metabolic rate?
BMR is what you would burn lying completely still, awake but not moving, in a thermally neutral room, having not eaten for twelve hours. It covers only the involuntary work of staying alive.
The organs dominate that bill. The brain alone accounts for roughly 20% of resting energy use, the liver another 20%, and the heart and kidneys about 10% between them. Skeletal muscle contributes less at rest than most people expect — around 20% despite being the largest tissue by mass.
This is why adding muscle raises BMR far less dramatically than fitness marketing suggests. A kilogram of muscle burns roughly 13 extra calories a day at rest, so a substantial 5 kg gain adds about 65 calories — real, but not transformative on its own.
BMR vs RMR vs TDEE
Three terms get used interchangeably and should not be.
| Measure | What it covers | Relative size |
|---|---|---|
| BMR | Complete rest, fasted, strict lab conditions | Baseline |
| RMR (resting metabolic rate) | Rest under everyday conditions | About 10% higher than BMR |
| TDEE | Everything — rest, movement, exercise, digestion | BMR × 1.2 to 1.9 |
In practice, BMR and RMR are close enough that most calculators and most research use them interchangeably. TDEE is the one that matters for planning what to eat, because it is the number you actually burn.
Which equation should I use?
- Mifflin-St Jeor — the default, and the right choice if you are unsure. Derived in 1990 from a more representative sample than its predecessor.
- Revised Harris-Benedict — the 1919 original as updated in 1984. Still widely cited, but tends to overestimate, particularly in people carrying more body fat.
- Katch-McArdle — driven by lean body mass instead of total weight, so it makes no assumption about your body composition. The most accurate of the three if you have a reliable body-fat measurement, and the least accurate if you are guessing.
The comparison table in your result shows all three side by side. When they cluster within 50–100 calories, any of them is fine. When Katch-McArdle diverges sharply from the other two, that is a signal your body composition is unusual for your weight — in which case Katch-McArdle is the one to trust, provided the body-fat figure is measured rather than estimated.
What changes your BMR
- Body size — larger bodies have more metabolically active tissue, which is why weight and height dominate every equation.
- Age — BMR declines roughly 1–2% per decade after 20, driven mostly by muscle loss rather than by metabolism 'slowing' as such.
- Sex — men average higher BMR at the same height and weight because they typically carry more lean mass.
- Thyroid function — hypothyroidism can lower BMR by 15–40%, hyperthyroidism raise it comparably. This is the most common medical reason a real measurement diverges sharply from a predicted one.
- Prolonged dieting — sustained energy restriction lowers BMR beyond what weight loss alone predicts, an adaptation that partly explains stalled weight loss.
Should I eat at my BMR?
No. Eating at BMR means eating as though you spent the entire day motionless, which nobody does. Even a fully sedentary day burns about 20% above BMR once you account for standing, walking, and digesting food.
Frequently asked questions
What is a normal BMR?
For adult women it typically falls between 1,200 and 1,600 calories a day, and for adult men between 1,600 and 2,000. Body size drives most of the variation, so a tall or heavy person will sit well above these ranges without anything being unusual.
Is BMR the same as calories burned per day?
No. BMR only covers resting function. Total daily energy expenditure adds movement, exercise, and digestion, and is typically 20% to 90% higher than BMR depending on how active you are.
Does BMR decrease with age?
Yes, by roughly 1–2% per decade after age 20. Most of that decline reflects lost muscle mass rather than a change in metabolism itself, which is why resistance training blunts it considerably.
Can I increase my BMR?
Somewhat. Adding muscle raises it by about 13 calories per kilogram of muscle per day, and avoiding prolonged severe restriction prevents it falling further than weight loss alone explains. Claims of large increases from foods or supplements are not supported.
Why is my BMR different on other calculators?
Different sites default to different equations. Harris-Benedict typically returns 5–10% higher than Mifflin-St Jeor for the same person. Check which equation a calculator uses before comparing results.
Should I eat below my BMR to lose weight?
Generally no. Eating below BMR is unnecessary because your actual daily burn is well above it, and sustained very low intakes make nutritional adequacy difficult and accelerate metabolic adaptation. A moderate deficit from TDEE works better.
Sources
- A new predictive equation for resting energy expenditure in healthy individuals — Mifflin et al., American Journal of Clinical Nutrition, 1990
- The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass — Roza & Shizgal, American Journal of Clinical Nutrition, 1984
- Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults — Frankenfield et al., Journal of the American Dietetic Association, 2005
- Dietary Reference Intakes for Energy — National Academies of Sciences, Engineering, and Medicine, 2023