Body mass index (BMI) Body mass index (BMI) = weight (kg)/height squared (square meter). The normal range is 18-25, below 18 is underweight, 25-28 is overweight, and above 28 is overweight. This formula applies to both men and women. Being overweight or underweight will increase the risk of illness, so the insurance industry usually charges extra for those whose body mass index is below normal or overweight. Fat and thin calculation formula-height and weight method male (height cm-80) × 0.7 = standard weight (kg) female (height cm-70) × 0.6 = difference between standard weight (kg) and standard weight; Underweight-below-10% of normal weight-10%. More than 20% (20%-39% mild obesity, 40%- 100% moderate obesity, 100% severe obesity)