We extend our previous calibration of the infrared Ca ii triplet (CaT) as a metallicity indicator to the metal-poor regime by including observations of 55 field stars with Fe/H down to −4.0 dex. While we previously solved the saturation at high metallicity using a combination of a Lorentzian and a Gaussian to reproduce the line profiles, in this paper we address the non-linearity at low metallicity following the suggestion of Starkenburg et al. of adding two non-linear terms to the relation among the Fe/H, luminosity and strength of the calcium triplet lines. Our calibration thus extends from −4.0 to +0.5 in metallicity and is presented using four different luminosity indicators: V − V HB, M V , M I and M K . The calibration obtained in this paper results in a tight correlation between Fe/H abundances measured from high-resolution spectra and Fe/H values derived from the CaT, over the whole metallicity range covered.