If you've ever wondered why two people the same weight can eat completely different amounts and both stay the same size, the answer is mostly BMR. It's set by your size, sex, age, and how much muscle you carry — and it accounts for two-thirds of every calorie you burn in a day.
In this article
What BMR actually measures
BMR (basal metabolic rate) is how many calories your body burns over 24 hours if you did absolutely nothing — lying still, awake but at full rest, in a thermally neutral room, fasted for 12+ hours. It's the cost of existing: keeping organs running, maintaining body temperature, repairing cells, thinking thoughts.
The breakdown of where BMR calories go (approximate, varies by person):
| System | % of BMR | kcal/day (1,600 BMR) |
|---|---|---|
| Liver | ~27% | ~430 kcal |
| Brain | ~19% | ~300 kcal |
| Muscles (at rest) | ~18% | ~290 kcal |
| Kidneys | ~10% | ~160 kcal |
| Heart | ~7% | ~110 kcal |
| Everything else | ~19% | ~310 kcal |
This is also why "boost your metabolism with X" claims rarely move the needle — your liver and brain are doing 46% of the work, and you can't speed them up with green tea.
The Mifflin-St Jeor formula (with example)
There are three commonly-cited BMR equations: Harris-Benedict (1919, revised 1984), Mifflin-St Jeor (1990), and Katch-McArdle (uses body fat percentage). Mifflin-St Jeor is the standard because it's the most accurate for the general population — within ±10% for most healthy adults.
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
Worked example. 35-year-old woman, 165 cm (5'5"), 68 kg (150 lb):
BMR = 680 + 1,031 − 175 − 161
BMR = 1,375 kcal/day
That's her resting calorie burn — what she'd need to eat to maintain weight if she spent the whole day in bed. Real maintenance is higher; that's where TDEE comes in.
BMR reference table by age & weight
Quick lookup for adults at average height (women ~165 cm / 5'5", men ~178 cm / 5'10"). Use this to sanity-check a calculator result.
| Age | Woman 60 kg / 132 lb | Woman 75 kg / 165 lb | Man 75 kg / 165 lb | Man 90 kg / 198 lb |
|---|---|---|---|---|
| 20 | 1,360 | 1,510 | 1,725 | 1,875 |
| 30 | 1,310 | 1,460 | 1,675 | 1,825 |
| 40 | 1,260 | 1,410 | 1,625 | 1,775 |
| 50 | 1,210 | 1,360 | 1,575 | 1,725 |
| 60 | 1,160 | 1,310 | 1,525 | 1,675 |
| 70 | 1,110 | 1,260 | 1,475 | 1,625 |
BMR vs TDEE vs RMR
Three terms get used somewhat interchangeably. They aren't the same.
| Term | What it measures | How big it is |
|---|---|---|
| BMR (basal metabolic rate) | Calories burned at complete rest, fasted, in a controlled lab setting | ~1,200–2,000 kcal/day |
| RMR (resting metabolic rate) | Calories burned at rest in real-world conditions (not fully fasted, room temp). About 5–10% higher than BMR. | ~1,300–2,200 kcal/day |
| TDEE (total daily energy expenditure) | BMR + activity + exercise + thermic effect of food. This is what you actually eat to maintain weight. | ~1,600–3,500+ kcal/day |
To get TDEE from BMR, multiply by an activity factor:
| Activity level | Description | Multiplier |
|---|---|---|
| Sedentary | Desk job, little or no exercise | BMR × 1.2 |
| Lightly active | Light exercise 1–3 days/week | BMR × 1.375 |
| Moderately active | Moderate exercise 3–5 days/week | BMR × 1.55 |
| Very active | Hard exercise 6–7 days/week | BMR × 1.725 |
| Extra active | Physical job + daily training | BMR × 1.9 |
Continuing the example above: 1,375 BMR × 1.375 (lightly active) = 1,890 kcal/day maintenance. That's the number that goes into a weight-loss or weight-gain calculation.
What actually changes your BMR
Bigger bodies burn more calories
More tissue = more cellular work = more calories. This is why heavier people burn more at rest, and also why BMR drops as you lose weight (you're maintaining less tissue). A 200 lb person who drops to 170 lb burns ~150 kcal/day less at rest just from the size change.
Muscle is more metabolically active than fat
Resting muscle burns about 6 kcal/lb/day; resting fat burns about 2 kcal/lb/day. Not a huge gap per pound, but if you gain 10 lb of muscle while losing 10 lb of fat, you net ~40 kcal/day more burn. Over a year that's 14,000 kcal — about 4 lb. Not magic, but real.
BMR drops ~1–2% per decade after 20
Most of this is muscle loss with age (sarcopenia), not a mysterious metabolic decline. Adults who keep lifting weights into their 60s show much smaller BMR drops than sedentary peers. The Pontzer et al. 2021 Science paper found BMR is actually flat from age 20 to 60 once you adjust for body composition.
Men burn ~5–10% more than women at the same weight
Mostly because men carry more muscle and less fat as a percentage of body weight. That's why the Mifflin-St Jeor formula subtracts 161 for women but adds 5 for men.
Chronic underfeeding lowers BMR
Long stretches in a steep calorie deficit cause adaptive thermogenesis — your body lowers BMR by 5–15% to conserve energy. This is the real reason crash diets stall and rebound. The fix isn't a faster metabolism; it's reverse-dieting back to maintenance for a few weeks before trying another deficit.
"BMR testing near me" — is it worth it?
You can pay to get BMR measured by indirect calorimetry — usually at a sports performance lab, university kinesiology clinic, or some metabolic-health practices. You breathe into a mask for 10–20 minutes; the device measures oxygen consumption and CO₂ output to calculate calorie burn at rest. Cost: typically $75–200 in the US.
Is it worth it? For most people, no. The Mifflin-St Jeor formula is within ±10% of measured BMR — and that ±10% is smaller than the day-to-day noise in your food logging accuracy. You'll get more out of two weeks of careful tracking + bodyweight trend than from one lab test.
It's potentially worth it if: (1) you have a medical condition affecting metabolism (thyroid, PCOS, post-bariatric), (2) you've been stuck at a weight plateau for months despite consistent tracking, or (3) you're an athlete tuning performance and the ±10% matters. Otherwise the formula does the job.
Using BMR to set a daily calorie target
BMR is a starting point, not the answer. Here's the workflow:
- Calculate BMR with Mifflin-St Jeor (or a calculator).
- Multiply by activity factor to get TDEE (maintenance calories).
- Adjust for your goal:
- Lose weight: TDEE − 250 to 500 kcal/day (½ to 1 lb/week loss)
- Maintain: eat at TDEE
- Gain weight: TDEE + 250 to 500 kcal/day (½ to 1 lb/week gain)
- Track for 2 weeks at the calculated target.
- Adjust based on weight trend — if the scale didn't move as predicted, your real TDEE is different from the formula. Add or subtract 100 kcal/day and try another 2 weeks.
Most people land within ±200 kcal of their formula prediction after one or two adjustment cycles. That's how you know it's your real number — not the calculator's guess, but the one that actually matches your body.
Frequently asked questions
What is BMR?
BMR (basal metabolic rate) is the number of calories your body burns at complete rest over 24 hours — just keeping organs, brain, and basic biology running. It excludes any movement, exercise, or digestion. For most adults, BMR accounts for 60–70% of total daily calorie burn.
How do I calculate my BMR?
The standard formula is Mifflin-St Jeor (1990).
Men: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5
Women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161
Plug in your weight in kilograms, height in centimetres, and age in years.
What is the difference between BMR and TDEE?
BMR is calories burned at rest. TDEE (total daily energy expenditure) is BMR plus everything else — walking, exercise, digesting food, fidgeting. TDEE is what you actually eat to maintain weight. You get TDEE by multiplying BMR by an activity factor (1.2 for sedentary, up to ~1.9 for very active).
How accurate is a BMR calculator?
Mifflin-St Jeor is accurate within ±10% for most healthy adults — typically within 200 kcal of measured BMR from indirect calorimetry. It slightly overestimates in people with low muscle mass and underestimates in very muscular individuals. For most people, the formula is close enough to use as a starting point and adjust based on real weight-change data.
Can I increase my BMR?
Modestly, yes. Adding muscle through resistance training is the most reliable lever — each pound of muscle adds roughly 6 kcal/day, so 10 lb of muscle is about 60 kcal/day. Larger gains come from eating enough (chronic underfeeding lowers BMR), getting enough sleep, and managing stress. Supplements, spicy food, and "metabolism boosters" have negligible effects.
Should I eat below my BMR to lose weight?
Generally no. Your weight-loss deficit should be subtracted from TDEE (maintenance), not from BMR. Eating below BMR for long stretches drives adaptive thermogenesis, muscle loss, hormonal disruption, and binge cycles. For most adults the practical floor is 1,200 kcal/day (women) or 1,500 kcal/day (men), and lower than that should be supervised.
BMR is the start. Tracking is what proves it.
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