What is BMR (basal metabolic rate)?

In one line

Basal metabolic rate is the minimum energy the body uses just to stay alive, even while resting and doing nothing.

What is BMR (basal metabolic rate)?
The number on every body composition report
Body composition reports and diet apps show 'BMR ○○ kcal'. People say it is the energy you burn even when you do nothing. What exactly does it mean?
In one line
Energy used just to stay alive
Basal metabolic rate is the minimum energy the body uses to stay alive, even when you lie still and do nothing. It is usually shown as calories (kcal) per day. BMR stands for 'Basal Metabolic Rate'.
What 'metabolism' means
Chemical reactions that get and use energy
The body breaks the nutrients in food into small pieces to get energy, and uses that energy to build what it needs. All of these chemical reactions inside the body together are called metabolism. BMR is the most basic energy this metabolism needs.
What it is used for
Heart, breathing, body heat, brain
Even lying still, your heart beats, you breathe and your brain works. The body also keeps its temperature and keeps replacing its cells with new ones. BMR is the energy for all of these jobs added together.
Who uses the most
Small but busy organs
The brain weighs only about 1.5 kg, but it uses about 20% of the energy the body uses at rest. At rest, the heart, kidneys and liver also use more than ten times as much energy as the same weight of muscle.
How it is measured
Calculated from your breath
It is measured in the morning after waking, lying down, after 10 to 12 hours without food and with the body rested, in a room that is neither hot nor cold. The energy used is calculated from how much oxygen you breathe in and carbon dioxide you breathe out. This is called indirect calorimetry.
A similar term
Resting metabolic rate (RMR)
Resting metabolic rate is measured under slightly looser conditions. It is taken after more than 7 hours without food and about 30 minutes of lying down, and it does not have to be in the morning. BMR differs in being measured right after waking in the morning, after a longer fast.
Why it differs between people
Body size and 'weight without fat'
BMR depends on age, sex, body size and what the body is made of. What explained the most of the difference between people was the weight of the parts that are not fat, like muscle, bone and organs. The same person's BMR was almost the same each time it was measured.
Its share of the day
The biggest piece
BMR makes up about 45-70% of all the energy used in a day, the biggest share. The rest is the energy used to digest food and to move the body. All three together are called TDEE (total daily energy expenditure).
Common myth
Is eating your BMR enough for the day?
No. BMR leaves out the energy used to digest food and move the body. Even people who spend most of the day sitting use about 1.4-1.7 times their BMR each day. What an adult needs in a day equals their TDEE.
Summary
The basic energy of staying alive
BMR is the minimum energy the body uses for the heart, breathing, body heat and brain, even at rest. What you need in a day is TDEE, which adds digestion and movement on top. Related terms: TDEE, calorie, resting metabolic rate.