Your basal metabolic rate (BMR) is the energy your body uses at complete rest. Multiply it by an activity factor and you get your total daily energy expenditure (TDEE) — roughly the calories needed to maintain your current weight.
The Mifflin–St Jeor equation
This calculator uses the Mifflin–St Jeor formula, widely considered the most accurate simple estimate. It combines your weight, height, age, and sex; the activity multiplier then scales BMR up to account for daily movement and exercise.
These are estimates, not prescriptions. Actual needs vary with body composition, genetics, and health. To lose weight, a moderate deficit below maintenance is typical — but consult a professional before making big changes.