Basal metabolic rate (BMR)
What BMR is, how it's calculated, and why it matters for weight management and health.
The calories you burn by just existing
Even if you stayed in bed all day, with no movement, no thinking, no stress, your body would still burn a significant number of calories just to keep the lights on. That's your Basal Metabolic Rate: the energy cost of breathing, pumping blood, maintaining body temperature and keeping cells alive. For most people, BMR accounts for 60 to 75 % of total daily calories burned, which makes it the largest single item in the energy budget, larger than any workout.
You will also meet the term resting metabolic rate (RMR). Strictly, BMR is measured after a night's sleep, 12 hours fasted, lying still in a temperature-neutral room; RMR relaxes those conditions and comes out a few per cent higher. Wearables, calculators and this page use the two words interchangeably, and the difference is smaller than the error of any estimate.
What drives it
- Lean mass, the biggest factor you can control. Muscle burns roughly 13 kcal per kilogram per day at rest against about 4.5 kcal for fat (Wang 2010). Organs burn far more per kilogram than either, which is why two people of the same weight can differ by a few hundred calories
- Age. BMR drops about 1 to 2 % per decade after your 20s, largely because of muscle loss. A large 2021 analysis of doubly labeled water data found that, adjusted for body composition, energy expenditure is actually stable from 20 to 60 and only then declines (Pontzer 2021). The age effect is mostly a lean-mass effect
- Sex. Men typically have a higher BMR because of greater muscle mass; at equal lean mass the gap nearly disappears
- Body size. Bigger bodies need more energy to maintain, and height matters as well as weight
- Thyroid, illness and fever. Hyperthyroidism raises it, hypothyroidism lowers it, and every degree of fever adds roughly 10 to 13 %
- Dieting. Prolonged calorie restriction lowers BMR beyond what the lost weight explains, the adaptive thermogenesis that makes the last kilograms slower than the first
The Mifflin-St Jeor formula
The most widely used estimate, and the one the American Dietetic Association found most accurate in non-obese and obese adults (Frankenfield 2005):
- Men: 10 x weight (kg) + 6.25 x height (cm) - 5 x age + 5
- Women: 10 x weight (kg) + 6.25 x height (cm) - 5 x age - 161
A 40-year-old man of 80 kg and 180 cm comes out at 800 + 1 125 - 200 + 5 = 1 730 kcal a day. The formula is right to within 10 % for about seven people in ten; the rest sit further out, usually because their lean mass is unusual for their weight. If you know your body fat percentage, the Katch-McArdle form (370 + 21.6 x lean mass in kg) removes that source of error.
How it is actually measured
The reference method is indirect calorimetry: breathe into a hood or mask for 20 to 30 minutes and the analyser reads oxygen consumed and carbon dioxide produced, which convert directly into calories. Sports clinics and some hospitals offer it. A metabolic cart is the only way to learn your own number rather than a population estimate.
Wearables do not measure BMR. Garmin, Polar and the rest compute it from the same kind of formula, using the age, sex, height and weight in your profile, and then add activity on top. If your profile weight is a year old, every calorie figure your watch shows inherits the error.
Why it matters
BMR is the floor under everything else. Total daily energy expenditure, the number weight management actually depends on, is BMR times an activity factor, which is the subject of the TDEE entry. Eating well below BMR for weeks is the pattern that costs muscle and slows the metabolism; eating at BMR with an active life is a deficit of several hundred calories a day without any counting.
In MyBodyAI
Basal calories come from your wearable when it reports them, because the device knows your profile and its own algorithm, and on one Garmin account that figure sat 381 kcal above the textbook estimate. A day the device has not covered yet, typically today and yesterday on a manual export, takes the median of the basal figures measured in the seven calendar days before it, and a carried figure that would exceed a finished day's total burn is refused rather than trusted. When those seven days hold no measurement, the Energy card falls back to Mifflin-St Jeor. Body composition from a connected scale sharpens the biological age model far more than it sharpens BMR, so that is where a smart scale earns its keep.
Frequently asked questions
How many calories is a normal BMR?
Most adults sit between 1 200 and 2 000 kcal a day. A 60 kg woman of 30 and 165 cm comes out near 1 320 kcal on Mifflin-St Jeor; a 90 kg man of 35 and 185 cm near 1 890 kcal. Lean mass moves the figure more than anything else, so two people of the same weight can differ by several hundred calories.
Can I raise my BMR?
Modestly, and mainly through muscle. Each kilogram of lean mass adds roughly 13 kcal a day at rest, so five kilograms of muscle built over a year is about 65 kcal a day, before counting the training itself. Caffeine and cold exposure produce small, short-lived rises. What lowers it reliably is prolonged crash dieting, which is worth avoiding for that reason alone.
Does my watch measure BMR?
No. Every consumer wearable calculates it from a formula using the age, sex, height and weight in your profile, then adds the activity it measures. Only indirect calorimetry in a clinic measures it. Keeping your profile weight current is the single most useful thing you can do for the calorie numbers on your wrist.