The International Research on the Interior of the Sun (IRIS) helioseismometer measures the full disk line of sight velocity of the Sun. In fact, it makes a photometric mesurement using two monochromatic spectral windows located on the wings of the D1 solar line. This is a non-linear measurement. Before a scientific exploitation of the IRIS data, the instrumental signal must be converted from non-linear photometric data into calibrated line of sight velocity. In this process, it is necessary to extract the small component due to the solar surface motions from the much larger contributions of the Earth spin, the Earth orbit, the gravitational redshift, the D1 line distortions produced by solar activity and even some telluric atmospheric effects. This paper describes the calibration method which is now used for pre-processing the IRIS data. It is the result of several iterations, and the use of one and a half years of IRIS data from one instrument, at Teide Observatory. It is certainly the best possible method to date, given the quality of the current data, and it can be regarded as valid over all the entire p-mode frequency range, and down to 100 microHz or so in the g-mode range. At lower frequencies, calibration, solar noise and merging techniques cannot be completely separated, and possible further improvements are still under investigation.