We have analysed Chandra/High Energy Transmission Gratings spectra of the X-ray emission line gas in the Seyfert galaxy NGC 4151. The zeroth order spectral images show extended H-and He-like O and Ne, up to a distance r ∼ 200 pc from the nucleus. Using the 1st order spectra, we measure an average line velocity ∼ −230 km s −1 , suggesting significant outflow of X-ray gas. We generated Cloudy photoionisation models to fit the 1st order spectra; the fit required three distinct emission-line components. To estimate the total mass of ionised gas and the mass outflow rates, we applied the model parameters to fit the zeroth order emission-line profiles of Ne IX and Ne X. We determined the total mass of ≈ 5.4× 10 5 M . Assuming the same kinematic profile as that for the [O III] gas, derived from our analysis of Hubble Space Telescope/Space Telescope Imaging Spectrograph spectra, the peak X-ray mass outflow rate was ≈ 1.8 M yr −1 , at r ∼ 150 pc. The total mass and mass outflow rates are similar to those determined using [O III], implying that the X-ray gas is a major outflow component. However, unlike the optical outflows, the X-ray outflow rate does not drop off at r > 100 pc, which suggests that it may have a greater impact on the host galaxy.