BMI is a fairly hit and miss calcualtion in lots of ways- it is really just a weight and height calculation and there are lots of places on the net you can get it calculated. For example, http://nhlbisupport.com/bmi/
Anything, or anyone, that knows your weight and height can calculate your BMI, that is come up with a number. But it is the interpretation of a BMI calculation that is more complicated, and that can depend on why you want to know in the first place BMI itself in terms of healthy weight is not so great at taking into consideration ethnic issues or 'short people' for example. I'm short and my BMI is fairly high, because of the ratio of my legs and torso.
Hope this helps! And how do I know this stuff- I used to work in an eating disorder clinic where I had to know lots about BMI