The upper bound estimation method is applied for estimating perturbations in reactor theory calculations. The results of this method are numerically compared with the regular first-order perturbation theory. It is concluded from the comparison that first-order perturbation in the integral formalism gives the best estimate for the changes in eigen-values. For those cases in which first-order perturbation yields low estimates, it is possible to obtain lower and upper bounds by applying both methods. A discussion of the possibility of obtaining a rough estimation to a perturbation without knowing the unperturbed regular and adjoint fluxes is presented and demonstrated with a numerical example.