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We investigate the relation between the optical extinction (A V ) and the hydrogen column density (N H ) determined from X-ray observations of a large sample of Galactic sightlines toward 35 supernova remnants, 6 planetary nebulae, and 70 X-ray binaries for which N H was determined in the literature with solar abundances. We derive an average ratio of N H /A V = (2.08 ± 0.02) × 10 21 H cm −2 mag −1 for the whole Galaxy. We find no correlation between N H /A V and the number density of hydrogen, the distance away from the Galactic centre, and the distance above or below the Galactic plane. The N H /A V ratio is generally invariant across the Galaxy, with N H /A V = (2.04 ± 0.05)×10 21 H cm −2 mag −1 for the 1st and 4th Galactic quadrants and N H /A V = (2.09 ± 0.03) × 10 21 H cm −2 mag −1 for the 2nd and 3rd Galactic quadrants. We also explore the distribution of hydrogen in the Galaxy by enlarging our sample with additional 74 supernova remnants for which both N H and distances are known. We find that, between the Galactic radius of 2 kpc to 10 kpc, the vertical distribution of hydrogen can be roughly described by a Gaussian function with a scale height of h = 75.5 ± 12.4 pc and a mid-plane density of n H (0) = 1.11 ± 0.15 cm −3 , corresponding to a total gas surface density of gas ∼ 7.0 M pc −2 . We also compile N H from 19 supernova remnants and 29 X-ray binaries for which N H was determined with subsolar abundances. We obtain N H /A V = (2.47 ± 0.04) × 10 21 H cm −2 mag −1 which exceeds that derived with solar abundances by ∼ 20%. We suggest that in future studies one may simply scale N H derived from subsolar abundances by a factor of ∼ 1.2 when converting to N H of solar abundances.