How the TDEE calculator works
The three burn components
TDEE = BMR (~60–75%) + thermic effect of food (~10%) + activity (the remainder, split between exercise and NEAT — the unconscious movement of daily life). NEAT is the wildcard: it varies up to 2,000 kcal/day between individuals and collapses silently during dieting, which is why plateaus arrive ahead of the math's schedule.
Why the formula needs two weeks of field testing
Mifflin-St Jeor lands within ~10% for most adults, but ±10% on 2,500 kcal is ±250 — the difference between a deficit and maintenance. The fix is empirical: eat at the calculated number for two weeks, watch the trend, adjust. The formula gets you to the neighborhood; the scale gets you to the house.
Choosing the activity factor honestly
The multipliers assume factor-consistent weeks: "moderate" (1.55) means 3–5 real workouts plus ordinary daily movement. Desk workers who train 4×/week are usually "light-to-moderate" (1.4 territory); manual laborers with no gym time can be "moderate." When torn between two levels, pick lower — the error costs less.