Your Total Daily Energy Expenditure (TDEE) is the total number of calories your body burns in 24 hours, including exercise, movement, and metabolic processes. Knowing your TDEE allows you to precisely set calorie targets for any goal: fat loss (deficit), muscle building (surplus), or maintenance (match). It's the starting point for all evidence-based nutrition planning.
TDEE is calculated from Basal Metabolic Rate (BMR) - the calories your body burns at rest - multiplied by an activity factor. The Mifflin-St Jeor equation is the most accurate BMR formula currently in use. Activity multipliers: sedentary (desk job, no exercise) x1.2; lightly active (exercise 1-3 days/week) x1.375; moderately active (exercise 3-5 days/week) x1.55; very active (hard exercise 6-7 days/week) x1.725; extra active (very hard exercise + physical job) x1.9.
Online TDEE calculators automate this calculation. However, all formulas are estimates - individual metabolic rates vary by 10-15%. The most accurate method is tracking: eat the same calories for 2-3 weeks while weighing yourself daily (average weekly weight). If weight is stable, that calorie intake is your actual TDEE. If gaining, lower by 100-150 calories. If losing and you don't want to, add 100-150 calories. Use your empirically determined TDEE rather than the formula estimate whenever possible, as it accounts for individual metabolic variation that no formula can capture.