Question

In order to calculate a person's bmi, you need to know the person's

Answer 1

In theory you need to know the persons hight and weight, however this is not the best measure for some people are just really muscular or there bones are a bit heavier. 

Answer 2

You need to know the person's height, weight, and the formula to calculate

BMI= kg/m^2

Your weight has to be in kilos
However, this isn't always accurate because a person can be all muscle and their BMI might consider them overweight. When in reality they aren't.