What BMR measures
Basal metabolic rate is the energy needed to keep you alive at rest: breathing, circulation, temperature control and tissue repair. It is measured under strict conditions, though most calculators approximate it with an equation using weight, height, age and sex. It depends mainly on body size and composition, and it falls as you lose weight.
What TDEE adds
Total daily energy expenditure is everything you burn in 24 hours. It starts with BMR, adds the energy used in daily movement and exercise, and includes the thermic effect of food — the cost of digesting what you eat, roughly 10% of intake. TDEE is what actually determines whether you gain, lose or maintain.
The activity multiplier
The bridge from BMR to TDEE is a multiplier that estimates how active you are. The common bands are 1.2 for sedentary, 1.375 for light activity, 1.55 for moderate, 1.725 for very active and 1.9 for extra active. Most people overestimate which band they belong in, which makes the calculated maintenance look too high.
Which one to use for what
Use TDEE for setting calorie intake — maintenance is TDEE, and a deficit or surplus is applied to it. Use BMR to understand your metabolic baseline and to see how it changes with weight and muscle. Eating at BMR alone would put you in a large, unsustainable deficit.
Why the estimates can be off
Equations predict within roughly 10–15% for an individual, and the activity multiplier is the weakest link because it relies on self-assessment. The fix is empirical: eat a consistent amount for two to three weeks and watch your weight. If it is stable, that intake is your true TDEE, whatever the formula said.