How is BMI calculated ? Explain the three steps taken

Body Mass Index (BMI):
Step 1 : Weight of person in kgand height in metres is taken.
Step 2 : Divide the weight by the square of the height.

Step 3 :
If BMI < 18.5, then person is undernourished.
If BMI > 25, then person is overweight. (Note : This criterion is not applicable to growing children)