BMI = weight ÷ height². BSA uses Mosteller: √(height in cm × weight in kg ÷ 3600). The band is read against the Asian cut-offs the whole way up — under 18.5 underweight, 18.5–22.9 normal, 23–24.9 overweight, 25 and above obesity — which is the WHO Asia-Pacific and 2009 Indian consensus scale, not the international one where 25 is overweight and 30 is obesity.
BMI says nothing about where the weight sits, and it reads high in a muscular adult and low in an older one who has lost muscle. It is a screening number, not a diagnosis.
It is a calculator, not a clinician. Nothing here is medical advice, and the person prescribing owns the decision.