Your basal metabolic rate, or BMR, is the number of calories your body burns just to keep you alive. It covers the basics: breathing, circulating blood, regulating temperature, and maintaining your cells. For most people, BMR accounts for 60% to 70% of all the calories burned in a day, making it by far the largest piece of your energy budget. The remaining calories go toward digesting food and physical activity.
Where Those Calories Actually Go
Your organs are surprisingly hungry. The brain alone burns about 240 calories per kilogram of its weight each day. The liver uses around 200, and the heart and kidneys each consume roughly 440 calories per kilogram daily. These four organs are relatively small compared to your total body weight, yet together they account for a large share of your resting calorie burn. Their metabolic rate is 15 to 40 times greater than an equivalent weight of muscle and 50 to 100 times greater than fat tissue.
Muscle still matters, though. A pound of muscle burns roughly 4.5 to 7 calories per day at rest. That’s modest on its own, but muscle makes up a much larger proportion of your body than any single organ. The more lean mass you carry, the higher your BMR tends to be. Fat tissue, by comparison, is far less metabolically active.
How to Estimate Your BMR
Two equations dominate BMR calculators online: the Harris-Benedict equation and the Mifflin-St Jeor equation. Both use your weight, height, age, and sex. The Mifflin-St Jeor formulas look like this:
- Men: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
- Women: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
Both equations perform reasonably well when averaged across groups. At the individual level, however, both can be off by a meaningful margin. A formula might estimate your BMR at 1,500 calories when a lab measurement shows 1,350 or 1,650. This is worth keeping in mind if you’re using a BMR number to set a calorie target: treat it as a starting estimate, not a precise reading.
BMR vs. Resting Metabolic Rate
You’ll often see BMR and RMR (resting metabolic rate) used interchangeably, and the two numbers are close. The difference is in how strictly they’re measured. A true BMR reading requires an overnight fast, no exercise for the previous 24 hours, a familiar testing environment, freedom from emotional stress, and complete rest. RMR is measured under more relaxed conditions, typically after at least 15 minutes of rest and 12 hours without exercise, but without all the strict controls. In practice, RMR tends to run slightly higher than BMR, usually by about 3% to 10%. Most online calculators and fitness apps actually estimate RMR, even when they call it BMR.
What Changes Your BMR
Body composition is the biggest lever. Because muscle tissue burns more calories at rest than fat tissue, two people who weigh the same can have noticeably different BMRs depending on how much of their weight is lean mass versus body fat. This is also why taller and heavier people generally have higher BMRs: they simply have more tissue to maintain.
Age plays a role, but probably less than you’ve been told. A large 2021 study published in Science found that metabolism holds remarkably steady between ages 20 and 60, regardless of sex. The real decline begins around age 60, when BMR drops by about 0.7% per year. By age 90 and beyond, adjusted total energy expenditure is roughly 26% below that of middle-aged adults. The decline tracks partly with the loss of lean mass that accelerates in older age, though some of the drop goes beyond what body composition alone would explain.
Thyroid function has a direct and sometimes dramatic effect. An underactive thyroid slows metabolism, which often shows up as fatigue, cold intolerance, constipation, and gradual weight gain. An overactive thyroid pushes metabolism higher, producing symptoms like unintentional weight loss, heat intolerance, and a fine tremor. If your weight is shifting in ways that don’t match your eating or activity habits, thyroid function is one of the first things worth checking.
Turning BMR Into Usable Calorie Needs
Your BMR tells you what your body burns at complete rest. To estimate total daily energy expenditure (TDEE), the number that actually reflects how many calories you use in a day, you multiply BMR by an activity factor:
- Sedentary (little or no exercise): BMR × 1.2
- Low active (light exercise 1–3 days/week): BMR × 1.4 to 1.5
- Active (moderate exercise 3–5 days/week): BMR × 1.6 to 1.7
- Very active (hard exercise 6–7 days/week or physical job): BMR × 1.9 or higher
Most adults fall into the sedentary or low active range, even if they exercise a few times per week. A desk job with evening workouts three days a week typically lands in the low active category. The resulting TDEE is your maintenance number: eating below it leads to weight loss over time, and eating above it leads to weight gain. Because BMR estimates can be imprecise, your TDEE calculation works best as a starting point that you adjust based on what actually happens on the scale over two to four weeks.
Why BMR Matters for Weight Management
Eating below your BMR for extended periods can work against you. When calorie intake drops too low, your body can adapt by slowing non-essential functions, reducing energy output in subtle ways like lower body temperature and less spontaneous movement. This is one reason very aggressive diets often stall. Knowing your approximate BMR helps you set a calorie floor: most approaches to sustainable fat loss aim for a daily intake somewhere between BMR and TDEE, creating a moderate deficit without triggering the strongest adaptive responses.
Building or preserving muscle through resistance training can nudge your BMR upward over time. The effect per pound of muscle is small, but over years it adds up, and the benefits extend well beyond calorie burn. Muscle mass is one of the few BMR-related factors you can directly control, making strength training one of the most practical tools for keeping your metabolism healthy as you age.