  We obtain a quantitative version of the classical Chevalley-Weil theorem for
curves. Let $\phi : \tilde{C} \to C$ be an unramified morphism of non-singular
plane projective curves defined over a number field $K$. We calculate an
effective upper bound for the norm of the relative discriminant of the number
field $K(Q)$ over $K$ for any point $P\in C(K)$ and $Q\in{\phi}^{-1}(P)$
