We report a simple, rapid, and quantitative wide-field technique to measure the optical extinction \(\sigma_{\rm ext}\) and scattering \(\sigma_{\rm sca}\) cross-section of single nanoparticles using wide-field microscopy enabling simultaneous acquisition of hundreds of nanoparticles for statistical analysis. As a proof of principle, we measured nominally spherical gold nanoparticles of 40\,nm and 100\,nm diameter and found mean values and standard deviations of \(\sigma_{\rm ext}\) and \(\sigma_{\rm sca}\) consistent with previous literature. Switching from unpolarized to linearly polarized excitation, we measured \(\sigma_{\rm ext}\) as a function of the polarization direction, and used it to characterize the asphericity of the nanoparticles. The method can be implemented cost-effectively on any conventional wide-field microscope and is applicable to any nanoparticles.