The overlooked burner
NEAT — walking, fidgeting, standing, chores — varies up to 2,000 kcal/day between people of similar size, dwarfing the 300–500 kcal a gym session burns. Two office workers with identical workouts can differ by a meal's worth of calories purely in daily movement.
Where it goes in a deficit
Dieting silently shaves NEAT first: less pacing, fewer standing breaks, elevator over stairs — the body defending energy with movements you never chose. A 500 kcal planned deficit can become 350 effective. This is why steps (a pedometer target of 8–10k) are the cheapest diet-sustaining technology ever invented.
Engineering it back
Walking meetings, standing calls, park-far parking, a step target that lives beside your calorie target. None of it is exercise; all of it is the variable that decides whether your calculated deficit survives contact with a real week. The TDEE calculator's activity factors try to price this — the pedometer keeps them honest.