Whether you want to lose, gain, or maintain weight, it all starts from one number: your total daily energy expenditure, or TDEE — the calories your body burns in a day.
Step 1: BMR
Your basal metabolic rate (BMR) is what you'd burn at complete rest. The Mifflin-St Jeor equation — the most accurate for most people — estimates it from your sex, age, height, and weight.
Step 2: Activity factor
Multiply BMR by an activity factor, from 1.2 for sedentary up to 1.9 for very active, to get your TDEE. The Calorie Calculator does both steps and shows targets for losing or gaining; the BMR Calculator isolates the resting number.
Step 3: Set a target
To lose weight, eat below your TDEE; to gain, eat above it. A deficit of about 500 calories a day yields roughly one pound of fat loss per week, since a pound of fat is about 3,500 calories. These are well-validated estimates — track your weight for two to three weeks and adjust the target based on what actually happens.