Total Daily Energy Expenditure is the single most important number in body composition. Every diet, every macro plan, every progress projection starts from TDEE — total calories burned in 24 hours including basal metabolism, activity, NEAT (non-exercise activity thermogenesis) and the thermic effect of food. Eating at TDEE keeps weight stable. Eating below drops fat (or muscle, if protein is low and training is absent). Eating above adds mass (fat or muscle, depending on training).

The math is straightforward: TDEE = BMR × activity multiplier. BMR comes from Mifflin-St Jeor or Katch-McArdle. Multipliers range from 1.2 (desk job, no exercise) to 1.9 (heavy manual labor plus daily hard training). Most office workers who train 3–5 times a week land at 1.55, but even that is an estimate — wearables typically overestimate by 20–30%.

The right test is empirical. Log calories honestly for 14–21 days at a fixed intake. If weight is stable, that intake IS your real TDEE — trust the scale, not the formula. Drop 15–20% for fat loss (500 kcal deficit ≈ 1 lb fat loss per week for most people). Add 10–15% for muscle gain in a novice, 5–10% for intermediate, 100–200 kcal for advanced — beyond that you just add fat.

TDEE changes over time and this catches almost everyone. Losing 20 lb drops BMR by ~150 kcal and NEAT by another 100–200 (you move less unconsciously). Your maintenance is now 300+ kcal lower than when you started. Without recalculating, the plateau is inevitable. Recalculate every 10 lb of change and every time activity shifts (injury, new job, new sport).

Combine TDEE with macro planning: lock protein at 1.6–2.2 g/kg (higher during cuts), split remaining calories between carbs and fat by preference and performance. Set a realistic timeline (0.5–1% body weight change per week), track for 2–3 weeks before adjusting, and let the number do the work. This is the entire nutritional playbook for 95% of body composition goals.