Question

how to calculate body mass index

Answer 1

BMI is your weight (in kilograms) over your height squared (in centimeters). Let's calculate, however, using pounds and inches. For instance, the BMI of a person who is 5'3" and weighs 125 lbs iscalculated as follows: Multiply the weight in pounds by 0.45 (the metric conversion factor)

Answer 2

You have to find out your weight. Let it be M kg.
You have to find out your height in meter. Let it be h meter.
Then BMI is calculated as:

$BMI = \frac{M}{h^2}$

Normal range for BMI is 18-25.
BMI below 18 is under-weight and above 25 is over-weight.