The KATRIN experiment aims to measure the neutrino mass with a sensitivity of m=0.2 eV/c² (90% C.L.). One parameter, the isotopic composition of the tritium gas source, is measured inline by Raman spectroscopy. The required measurement trueness of the composition is <10%. Thus, accurate calibration of the Raman system is required. This is successfully obtained by two independent calibration methods: a gas sampling technique and an approach via theoretical intensities plus spectral sensitivity.