The molecular gas mass in nearby galaxies is generally estimated using 12CO(1-0) line intensities and assuming the X conversion factor between I(CO) and N(H2) measured in the solar neighborhood. It is however known that this X conversion factor is not universal since it changes with metallicity, cosmic ray density and UV radiation field. Far-IR data in the spectral range 100-1000 μm can be used to estimate the molecular gas content of late-type galaxies in an independent way of CO line measurements once a metallicity-dependent dust to gas ratio is assumed, allowing a direct estimate of X. This exercise is presented here for a large sample of galaxies with available multifrequency data using ISOPHOT and IRAS far-IR data. X spans from ~1020 mol cm-2 (K km s-1)-1 in giant spirals to ~1021 mol cm-2 (K km s-1)-1 in dwarf irregulars. The relationships between X and the UV radiation field (as traced by the Hα[NII]E.W.), the metallicity and the H band luminosity are analysed.