Body Mass Index (BMI) is a way to categorize oneself based on weight and height. It's very useful for a general population, and indeed, has been shown to be a good predictor of obesity related morbidity - meaning that the higher your BMI the greater your morbidity, as well as your risk of type 2 diabetes, cardiovascular disease and stroke. BMI is very easy to calculate and can quickly be used to determine your weight class using the following table:
Here's what I want to point out. BMI is great for the general population, however for an athlete you can basically throw it out the window. Marathon runners will commonly come in as underweight, olympic weight lifters or even football players will often come in as over weight, and body-builders are obese. However, no reasonable person would say that these people are unhealthy. BMI also does not take into account gender. Think about a man and woman of the same height. you would expect the man to weight more than the woman based on body-type, but according to BMI there should not be a difference. It also doesn't take into account a persons morphology (body type).
What I'm trying to say is take BMI with a grain of salt. Yes it is a good general estimate about your weight as it relates to your health, but it isn't be-all-end-all. Don't get to worried if you're tipping a little into the over weight category if you're an active individual who eats well. Do worry if you're in the overweight range and you have type 2 diabetes and smoke. Do you see the difference?