The BMI calculator from the website of the national heart lung and blood institute
Calculate your BMI - Standard BMI Calculator
Gives these ranges:
* Underweight = <18.5
* Normal weight = 18.5-24.9
* Overweight = 25-29.9
* Obesity = BMI of 30 or greater
Like others have said BMI can be quite limited as people with lots of muscle will be catagorised as obese and frail people could be seen as under-weight. There is a link on that website about the limitations of BMI.
Hope that helps
