The local surface plasmon resonance (LSPR) of noble metal nanoparticles has recently been exploited in numerous applications. The LSPR peak position and linewidth have been studied quite extensively, but the magnitude of the resonance has not received much attention. Analytical solutions to Maxwell's Equations cannot predict the scattering cross section of arbitrarily-shaped particles at arbitrary illumination and detection angles. Dark field microscpectroscopy is a powerful tool for studying plasmon resonances of noble metal nanoparticles and for developing their applications in sensing and imaging. We present a technique for calibrating dark field microspectrometer measurements to yield quantitative spectral scattering cross sections for arbitrarily shaped particles. Values for gold nanorods and gold bipyramids are reported. The measurements suggest that, for small elongated particles, the signal can be predicted by approximations based on the total cross section.