We present a new method for determining absolute values of quantum yield of luminescent emitters, which is based on the modification of the radiative transition of emitters within a tunable metallic nanocavity. The method presented is easy to set up and works without any calibration. It will thus be useful for all applications where absolute and calibration-free measurements of luminescence quantum yields are needed. Moreover, it requires only a minute amount of low-concentration fluorophore solution. We give a detailed description of the theory and data evaluation of the nanocavity measurements, and report experimental results for several common dyes in aqueous solution.