The engine idling
BMR — Basal Metabolic Rate — is the number of calories your body burns at complete rest: lying still, awake, fasted, in a neutral-temperature room. It's the cost of running your organs, maintaining body temperature, and replacing cells.
For most adults BMR is 1,200–2,000 kcal/day and represents 60–70% of everything they burn. Your brain alone claims about 20% of it; the liver another 20%; skeletal muscle roughly 20% even when totally inactive.
What determines your BMR
Body size dominates — more tissue costs more energy, which is why every BMR equation starts with weight and height.
Lean mass matters per-kilogram: muscle burns roughly three times more at rest than fat tissue, which is why two people of identical weight can have meaningfully different BMRs.
Age lowers BMR gradually — mostly through muscle loss rather than mysterious metabolic decay; landmark 2021 research in Science found metabolism is remarkably stable from age 20 to 60 once body composition is accounted for.
Sex shows up in equations because men carry more lean mass on average at a given height and weight.
Measured vs estimated
True BMR is measured with indirect calorimetry in a lab. Everyone else uses prediction equations — Mifflin-St Jeor for the general population, Katch-McArdle if you know your body-fat percentage — which land within about 10% for most people.
Note the distinction from RMR (Resting Metabolic Rate), measured under looser conditions and typically a few percent higher. Calculators use the terms loosely; the practical difference rarely matters.
Put it into practice: run your own numbers with the related calculator — free, instant, and nothing leaves your browser.
Frequently asked questions
Can I increase my BMR?
Meaningfully, only by building muscle — each kilogram adds roughly 10–15 kcal/day at rest. Crash dieting does the opposite, lowering BMR by shedding lean mass.
Why is eating below BMR considered risky?
It isn't inherently dangerous short-term, but aggressive intakes far below BMR usually mean inadequate protein and micronutrients, faster muscle loss, and rebound — supervision is wise.
Is a slow metabolism why I can't lose weight?
Rarely. Measured BMR differences between similar people are smaller than assumed; the variance lives mostly in NEAT and underestimated intake.
More guides
- What Is TDEE?
- Mifflin-St Jeor Calculator
- Harris-Benedict Calculator
- Katch-McArdle Calculator
- BMR vs TDEE: What's the Difference?
- What Are MET Values?
- What Is NEAT?
- The Thermic Effect of Food
- Activity Multipliers Explained
- What Is a Calorie?
- Metabolic Adaptation Explained
- The Calorie Deficit, Explained
- 500-Calorie Deficit
- 1,000-Calorie Deficit
- 300-Calorie Deficit
- How Many Calories to Lose 1 Pound a Week
- How Many Calories to Lose 2 Pounds a Week
- Reverse Dieting
- Maintenance Phase
- Calorie Cycling
- How Accurate Are TDEE Calculators?
- Not Losing Weight in a Calorie Deficit? 7 Real Reasons
- Is 1,200 Calories a Day Safe?
- Should You Eat Back Exercise Calories?
- How Long Does Metabolic Adaptation Last?
- Do You Burn Fewer Calories as You Lose Weight?
- Why Your Maintenance Calories Keep Changing
- Calorie Cycling vs Flat Deficit
- How to Avoid Muscle Loss on GLP-1 Medications
Sources
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990. [link]
- Harris JA, Benedict FG. A Biometric Study of Human Basal Metabolism. PNAS. 1918. [link]
- Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. Am J Clin Nutr. 1984. [link]