For adults (aged 20 or above):
http://www.nhlbisupport.com/bmi/
* Underweight = Below 18.5
* Normal weight = 18.5-24.9
* Overweight = 25-29.9
* Obesity = BMI of 30 or greater
For teens and children:
http://apps.nccd.cdc.gov/dnpabmi/Calculator.aspx
Body Mass Index can calculate if you're underweight, overweight or average by comparing your height and weight. If you want to input the numbers in the metric system, just click where it says metric on that site. It's right above where you input the numbers.
Now, many of you may get underweight, but that's because most of you are still growing. Also, a stupid number shouldn't mean anything anyway so don't worry. As long as you're comfortable in your body then it really doesn't matter. I've pretty much been rated as underweight all my life, but I didn't really think so. So I think this is not going to be accurate for many people anyway.
So post your number if you want and/or discuss whether you are happy with yourself.
I find myself to be a bit skinny yet, but it's not bothering me much. And my number is 21,4.
edit: oi, found a calculator for teens and children.