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 is the cost of existing: keeping organs running, maintaining body temperature, repairing cells, thinking thoughts.
Where those calories go, approximately:
| 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 cannot speed them up with green tea.
The Mifflin-St Jeor formula
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 is the most accurate for the general population — within ±10% for most healthy adults.
- Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
- Women: 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 = 1,375 kcal/day. That is her resting calorie burn — what she would need to eat to maintain weight if she spent the whole day in bed. Real maintenance is higher; that is where TDEE comes in.
BMR reference table by age and 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 RMR vs TDEE
Three terms get used somewhat interchangeably. They are not the same.
| Term | What it measures | How big it is |
|---|---|---|
| BMR | Calories burned at complete rest, fasted, in a controlled lab setting | ~1,200–2,000 kcal/day |
| RMR | Calories burned at rest in real-world conditions. About 5–10% higher than BMR. | ~1,300–2,200 kcal/day |
| TDEE | BMR + activity + exercise + thermic effect of food. What you actually eat to maintain weight. | ~1,600–3,500+ kcal/day |
To get TDEE from BMR, multiply by an activity factor from 1.2 (sedentary) to 1.9 (physical job plus daily training). The TDEE calculator does that step and shows the goal ranges that follow.
What actually changes your BMR
- Body size. More tissue means more cellular work. This is why heavier people burn more at rest, and why BMR drops as you lose weight — a 200 lb person who drops to 170 lb burns roughly 150 kcal/day less at rest from the size change alone.
- Muscle mass. Resting muscle burns about 6 kcal/lb/day; resting fat burns about 2. Gaining 10 lb of muscle while losing 10 lb of fat nets around 40 kcal/day more burn. Not magic, but real.
- Age. BMR drops roughly 1–2% per decade after 20, mostly from muscle loss rather than a mysterious metabolic decline. Pontzer et al. (2021, Science) found BMR is essentially flat from 20 to 60 once you adjust for body composition.
- Sex. Men burn about 5–10% more than women at the same weight, mostly from carrying more muscle and less fat proportionally. That is why the formula subtracts 161 for women and adds 5 for men.
- Eating enough. Long stretches in a steep deficit cause adaptive thermogenesis — the body lowers BMR by 5–15% to conserve energy. The fix is not a faster metabolism; it is returning to maintenance for a few weeks before attempting another deficit.
Is paid BMR testing worth it?
You can have BMR measured by indirect calorimetry, usually at a sports performance lab or university kinesiology clinic. You breathe into a mask for 10–20 minutes while the device measures oxygen and CO₂ to calculate resting burn. Cost is typically $75–200 in the US.
For most people it is not worth it. Mifflin-St Jeor lands within ±10% of measured BMR, and that margin is smaller than the day-to-day noise in food logging accuracy. Two weeks of careful tracking plus a bodyweight trend will teach you more than one lab test. It is potentially worth it if you have a condition affecting metabolism (thyroid, PCOS, post-bariatric), you have been stuck at a plateau for months despite consistent tracking, or you are an athlete for whom the ±10% genuinely matters.
Using BMR to set a daily target
BMR is a starting point, not the answer. The workflow:
- Calculate BMR with the tool above.
- Multiply by your activity factor to get TDEE.
- Adjust for your goal: TDEE − 250 to 500 to lose, TDEE to maintain, TDEE + 250 to 500 to gain.
- Track for two weeks at that target, then adjust from the weight trend. If the scale did not move as predicted, your real TDEE differs from the formula — shift by 100 kcal/day and try another two weeks.
Most people land within ±200 kcal of the formula prediction after one or two adjustment cycles. That is how you find your real number: not the calculator's guess, but the one that matches your body.