# How to get the black hole spin in RPM if you have the value of $a$?

Most research papers use the variable "a" to describe black hole spin, which always has a value of less than 1, I don't know what this variable is called, but from what I know it can't exceed 1 because you can't have a "naked black hole". Does this variable have a name? and is there a way to convert it into RPM ?

• Yes, there is just look up some books on it. If you show some of your work trying someone may help. – Bob Bee May 10 '17 at 2:44
• @Bob Bee The problem is I don't know what this variable is called, it makes it difficult to look it up. It's everywhere in journals but they don't mention its name. – user43783 May 10 '17 at 3:09
• Books, not journals. Or wiki articles or tutorials. I don't know the name if any, but I think it's angular momentum divided by total energy, with some factors to make it unitless. I'd think anywhere or some places they derive or work with the Kerr metric it'll have something. – Bob Bee May 10 '17 at 3:31