  We develop methods to extract resonance widths from finite volume spectra of
1+1 dimensional quantum field theories. Our two methods are based on Luscher's
description of finite size corrections, and are dubbed the Breit-Wigner and the
improved "mini-Hamiltonian" method, respectively. We establish a consistent
framework for the finite volume description of sufficiently narrow resonances
that takes into account the finite size corrections and mass shifts properly.
Using predictions from form factor perturbation theory, we test the two methods
against finite size data from truncated conformal space approach, and find
excellent agreement which confirms both the theoretical framework and the
numerical validity of the methods. Although our investigation is carried out in
1+1 dimensions, the extension to physical (3+1) space-time dimensions appears
straightforward, given sufficiently accurate finite volume spectra.
