Calculate the number of times to divide by two

Greetings.

I have a java method that I consider expensive, and I'm trying to replace some calls to it with a mathematical expression. Problem is, I suck at math. I mean really suck.

The following should explain the pattern that I'm trying to exploit.

f(x)   -> y
f(x*2) -> f(x)+1

That is, whenever I double the value for x, the value for y will be 1 greater than for x/2. Here are some example output:

f(5)   -> 6
f(10)  -> 7
f(20)  -> 8
f(40)  -> 9
f(80)  -> 10
f(160) -> 11
f(320) -> 12

My current approach is brute force. I'm looping over the X and test how many times I can halve it before I reach 5, and finally I add 6. This works and is faster than the call to the original method. But I was looking for a more "elegant" or potentially cheaper solution.

Accepted answer goes to the one who manages to help me without pointing out how stupid I am :)

(the title probably sucks, because I don't know what I'm looking for)