  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.
