Even if you spent a whole day lying still, your body would still burn a substantial amount of energy. Your heart beats, your lungs breathe, your brain works and your cells constantly repair themselves.
The energy needed for all of this is your Basal Metabolic Rate, or BMR. Understanding it is the first step to understanding your calorie needs.
BMR in plain English
BMR is the number of calories your body needs to maintain essential functions at complete rest. Strictly, it is measured in a lab after an overnight fast, in a warm room, lying still — conditions that are hard to reproduce, which is why most people use a prediction equation instead.
You may also see RMR (resting metabolic rate). RMR is measured under slightly less strict conditions and is usually a little higher than BMR, but in everyday use the terms are often treated as interchangeable.
What affects your BMR?
- Body size: larger bodies use more energy.
- Lean mass: muscle and organs use more energy than fat tissue, so more lean mass means a higher BMR.
- Age: BMR tends to decline gradually through adulthood, partly because of changes in muscle mass.
- Sex: men typically have a higher BMR than women of the same size, mainly because of differences in body composition.
- Other factors: genetics, hormones (such as thyroid hormones), illness, some medications and pregnancy.
How BMR is estimated
The Mifflin–St Jeor equation, published in 1990, is widely considered the most reliable of the common equations for healthy adults:
Example: a 30-year-old man, 180 cm and 80 kg: 800 + 1,125 − 150 + 5 = 1,780 kcal/day.
Other options include the revised Harris–Benedict equation (1984) and Katch–McArdle, which uses lean body mass and needs a body-fat percentage. The BMR calculator lets you compare all three.
BMR is not your calorie target
A common mistake is to treat BMR as the number of calories to eat. BMR leaves out all movement and exercise, as well as the energy used to digest food. Your total daily needs — your TDEE — are higher.
| BMR | TDEE | |
|---|---|---|
| What it covers | Energy at complete rest | All energy used in a day |
| Includes activity? | No | Yes |
| Typical use | Starting point for calculations | Basis for calorie targets |
Read What is TDEE? to see how BMR is turned into a daily calorie estimate.
Can you change your BMR?
Building and keeping muscle through regular strength training can support a modestly higher BMR over time, and it has many other benefits for health and daily function. Beyond that, the biggest lever you control is overall daily activity rather than BMR itself.
Try the calculators
Put this guide into practice with a free estimate.
Frequently asked questions
Is a higher BMR better?
Not in itself. A higher BMR simply means your body uses more energy at rest, usually because it is larger or has more lean mass. What matters for weight is the balance between total energy used and eaten.
What is the difference between BMR and RMR?
Both describe resting energy use. BMR is measured under stricter conditions; RMR is usually slightly higher. For everyday calculations the difference is small.
Does eating less lower your BMR?
Losing weight, and particularly losing muscle, lowers energy needs, and the body can also adapt to a prolonged deficit. Eating enough protein and strength training help limit muscle loss.
Methods and sources
- Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241–247.
- Roza AM, Shizgal HM. The Harris Benedict equation reevaluated. Am J Clin Nutr. 1984;40(1):168–182.
- Frankenfield D, et al. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc. 2005;105(5):775–789.