In this paper we generalize the notion of $n$-equivalence relation introduced by Chen et al. in \cite{Chen2014} to classify constacyclic codes of length $n$ over a finite field $\mathbb{F}_q$, where $q=p^r$ is a prime power, to the case of skew constacyclic codes without derivation. We call this relation $(n,σ)$-equivalence relation, where $n$ is the length of the code and $ σ$ is an automorphism of the finite field. We compute the number of $(n,σ)$-equivalence classes, and we give conditions on $ λ$ and $μ$ for which $(σ, λ)$-constacyclic codes and $(σ, λ)$-constacyclic codes are equivalent with respect to our $(n,σ)$-equivalence relation. Under some conditions on $n$ and $q$ we prove that skew constacyclic codes are equivalent to cyclic codes. We also prove that when $q$ is even and $σ$ is the Frobenius autmorphism, skew constacyclic codes of length $n$ are equivalent to cyclic codes when $\gcd(n,r)=1$. Finally we give some examples as applications of the theory developed here.