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total daily energy expenditure
TDEE multiplies your Mifflin-St Jeor BMR by an activity factor covering everything beyond lying still: work, walking, training and digestion. The multipliers (1.2 sedentary to 1.9 for athletes) are the standard Harris-derived bands — and the most common error is optimism, since a desk job plus three gym sessions is 'light-to-moderate', not 'very active'. When in doubt, pick the lower band and let results correct you.
TDEE is a starting estimate with honest ±10% error; your body runs the real experiment. Eat the maintenance figure for two or three weeks while weighing consistently: stable weight confirms it, drift reveals your true number at roughly 1,100 kcal per kilogram of monthly change. The goal adjustments apply the standard 500-kcal deficit (about 0.45 kg a week) or a lean 300-kcal surplus — small enough to add mostly muscle when training supports it.