The Body Mass Index (BMI) is a tool that can be used to tell how healthy a person's weight is.
You can use the BMI to find out if you're a healthy weight for your height.
  

                            weight in kilograms
   BMI     =       ———————————
( kg/m² )                height in meters²

 

For example, if you weigh 70kg, and you're 1.75m tall,
(70/ (1.75 x 1.75) = 22.9).
Your BMI would be 22.9


BMI Categories:
Underweight = <19.9
Normal weight = 20-24.9
Overweight = 25-29.9
Obesity = BMI of 30 or greater


Please note problem with BMI is that it does not differentiate lean muscle mass from fat mass, or even fat distribution.
BMI should be taken into account with your body fat percentage, as well as your hip to waist ratio. This will give you a more accurate picture of your body composition.