TDEE (Total Daily Energy Expenditure) is the number of calories your body burns in a day, including basal metabolism, digestion, daily movement, and exercise. Knowing your TDEE is the foundation of every successful nutrition plan — whether you want to lose fat, gain muscle, or maintain weight. This guide explains exactly how to calculate TDEE using the Mifflin-St Jeor equation, which activity multiplier to choose, and how to translate the result into a calorie target you can actually follow.