We provide accurate empirical relations for characterising a single-mode matched clad fiber from its far-field radiation pattern. The empirical relations can be used to calculate accurately the Petermann 2, the rms and d x mode field diameters of the fiber. For a measurement error of < 2' in the far-field angles 0 h , Θ 40 or θ χ , where 0 h is the half intensity angle and Θ 40 (θ χ ) corresponds to the -40 dB (-oo dB) intensity level, the empirical relations are found to give an error of