How to calculate body mass index Body mass index (BMI) = Weight (kg) / Height (m) squared BMI = Wt (Kg) / Ht (m2) For example: child weighs 50kg, height 1.50m BMI = 50 ÷ 1.5 ÷ 1.5 = 22.67 (Kg/m2) In comparison with the standards of the child’s body mass index (see the article on this site), you can determine whether the child is overweight or obese.