The ideal BMI for lean is calculated by taking your weight in kilograms and dividing it by your height in square meters. This formula will provide you with a ratio called the body mass index which can be interpreted as “underweight”, “normal”, “overweight” or “obese”. T o have a lean BMI, aim to score within the range of 18.5 -24.9 depending on your body type and age group. Above this range is considered overweight, while anything below it is classified as underweight.