What is TDEE (total daily energy expenditure)?

In one line

TDEE is the total amount of energy the body uses in a day by resting, eating and moving.

What is TDEE (total daily energy expenditure)?
The number every diet calculator shows
Type your height and weight into a diet calculator and it shows 'TDEE ○○ kcal'. What is this number that always comes up when people talk about how much to eat in a day?
In one line
All the energy the body uses in a day
TDEE stands for 'Total Daily Energy Expenditure': all the energy the body uses in one day added together. It is measured in calories (kcal). For an adult, eating this much each day keeps body weight the same.
It has three parts
Resting + digesting + moving
TDEE is mainly three parts added together: the energy used even while still, the energy used to digest food, and the energy used to move the body. The next cards look at each one.
First, basal metabolic rate
The biggest share, 45-70% of the day
The chemical reactions in the body that break nutrients down to get and use energy are called metabolism. The minimum energy this metabolism needs for the heart, breathing, body heat and brain, even when you do nothing, is the basal metabolic rate. It is the biggest of the three parts.
Second, the thermic effect of food
About 10% of the calories you eat
After you eat, the body also uses energy to digest food, take it into the body and store it. This is called the thermic effect of food (TEF). With a normal meal it is about 10% of the calories eaten; it is large for protein and small for fat.
Third, activity
The part that changes the most
This is the energy used to move the body. It is the second-biggest part after BMR, and the part that changes the most from person to person and from day to day. On days you move a lot TDEE goes up, and on days you move little it goes down.
How it is measured
Drink special water and watch for 1-3 weeks
The most accurate method is doubly labelled water. You drink water containing slightly heavier hydrogen and oxygen, and the energy used is calculated from the difference in how fast the two leave the body, measured in urine and other samples. It is measured over 1-3 weeks while you live as usual.
How it is estimated
BMR × activity level
Doubly labelled water is expensive and hard to handle, so TDEE is usually estimated by multiplying BMR by an 'activity level'. The activity level is how many times bigger the day's total is than BMR. A mostly sitting lifestyle is about 1.4-1.7 times.
Try the math
A person with a BMR of 1,290 kcal
If a person with a BMR of 1,290 kcal mostly sits and has an activity level of 1.53, their TDEE is about 1,975 kcal. If the same person lives a much more active life, this number goes up.
Why it matters
A scale between eating and using
If the calories you eat equal your TDEE, body weight stays the same. If you eat more, the leftover energy is stored mostly as fat; if you eat less, the body takes out the energy it has stored.
Common myth
No exercise means no activity calories?
No. Everyday movement that is not exercise, like walking, standing, housework and commuting, also uses energy. This is called non-exercise activity thermogenesis (NEAT), and it makes up most of the energy a person uses apart from resting.
Summary
Resting + digesting + moving = TDEE
TDEE is the day's total use of energy: BMR, the energy to digest food, and the energy to move, all added up. Even without exercise, everyday movement creates activity calories. Related terms: basal metabolic rate, calorie, NEAT.