Motivated by the concept of clean index of rings of Lee and Zhou we introduce the concept of nil clean index of rings. For any element a of a ring R with unity, we define η(a) = {e ∈ R | e 2 = e and a−e ∈ nil(R)}, where nil(R) is the set of all nilpotent elements of R. Then nil clean index of R is defined by sup{ |η(a)| : a ∈ R} and it is denoted by Nin(R). In this article, we characterize rings of nil clean indices 1, 2 and 3 and prove some interesting results pertaining them.