Whether you're looking to lose fat or build muscle, fitness goals always trace back to thermodynamics: energy inputs versus energy outputs.
BMR vs. TDEE
Your Basal Metabolic Rate (BMR) is the number of calories your body burns to survive in a resting state. Your Total Daily Energy Expenditure (TDEE) adds physical activity levels (exercise, work, walking) onto your BMR.
To lose weight, you must maintain a calorie deficit below your TDEE. To gain weight, you must maintain a calorie surplus.
Common Formula Standards
Standard calculators use the Harris-Benedict or Mifflin-St Jeor formulas to estimate resting metabolism based on weight, height, age, and gender parameters. Tracking metrics visually keeps workouts aligned.