In situ gamma spectrometry is a valuable tool to assess the radionuclides released in the environment and the associated dose. This requires prior establishment of coefficients allowing the conversion of the specific activity into ambient equivalent dose. The aim of this work is to calculate updated conversion factors for monoenergetic photons and for a series of radionuclides of interest. The calculation was performed using the Monte Carlo (MC) method, the GEANT4 MC code, various activity distribution models and up-to-date nuclear decay data. A new set of conversion factors is established in the energy range extending from  <100 keV to 8.5 MeV. The coefficients calculated in this work were compared to the data published in the literature.