Energy balance — the relationship between calories in and calories out — is the foundation of body composition. Gain weight when you eat more than you burn, lose weight when you eat less. While this is a simplification, it's accurate enough to be useful for most people most of the time.
Understanding Energy Expenditure
Your total daily energy expenditure (TDEE) has three main components. Basal Metabolic Rate (BMR) is the calories your body burns at complete rest — accounts for roughly 60-75% of total expenditure, determined primarily by weight, age, and sex. The Mifflin-St Jeor equation is the most accurate formula. Thermic Effect of Food (TEF) is the energy used to digest and process nutrients — protein has the highest TEF, fat the lowest. Physical Activity is the most variable component, ranging from 200 calories for a desk worker to 1000+ for an active laborer.
Activity Multipliers
Multiply BMR by an activity multiplier. Sedentary: 1.2. Lightly active: 1.375. Moderately active: 1.55. Very active: 1.725. Extra active: 1.9. Most people overestimate their activity level. Conservative estimates prevent the mysterious weight gain that comes from overestimating.
Adjusting Based on Real Results
Calculators give estimates, not certainties. The only way to truly know your needs is to track your intake and monitor your weight over 4-6 weeks. If you're gaining faster than desired at estimated maintenance, you're above maintenance. Adjust by 200-300 calories and reassess.