We report on an optical microscopy technique aimed at measuring the absolute absorption cross section of individual nanoparticles. It relies on the thermally induced variation of the refractive index of the surrounding medium subsequent to light absorption by the nanoparticle. The technique is illustrated on gold nanoparticles featuring a well-defined plasmonic resonance. Different plasmonic modes were evidenced and quantified. The simplicity and rapidity of the measurements make it possible to investigate absorption resonances of absorbing nanostructures and microstructures in a reasonable time frame. Furthermore, the experimental approach is simply based on the use of a wavefront analyzer, which is straightforward to implement on any conventional microscope.