Estimate your daily calories to maintain, lose or gain weight.
Estimate your daily calorie needs (TDEE) using the Mifflin-St Jeor equation. Enter your age, sex, weight, height and activity level to see how many calories you need to maintain, and targets to lose or gain about half a kilo per week.
Total Daily Energy Expenditure — the calories you burn per day including activity. Eating that amount maintains your weight.
Eat below your maintenance calories — a ~500 kcal daily deficit is roughly ½ kg per week. Consult a professional for medical advice.