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

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. 

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.

It should be around march 30th but there's no way to tell for sure. especially if your just starting. the may not come again for a few months.