For clarity, BMI (Basal Metabolic Index) is a way of relating your height and weight to know if you are underweight, normal weight, overweight or obese.
BMI= weight (kg) / height (m) x height (m).
BMI can be a very inaccurate way to measure body fat, as it only takes into consideration height and weight. Here is an example of a mass of fat vs a mass of muscle, both weighing 2.3kg (5lbs).