Context : Deriving the metallicity , [ Fe/H ] , in low-resolution spectra of carbon-enhanced metal-poor ( CEMP ) stars is a tedious task that , owing to the large number of line blends , often leads to uncertainties on [ Fe/H ] exceeding 0.25 dex .
The CEMP stars increase in number with decreasing [ Fe/H ] and some of these are known to be bona fide second generation halo stars .
Hence , knowing their [ Fe/H ] is important for tracing the formation and chemical evolution of the Galaxy .

Aims : Here , we aim to improve the [ Fe/H ] measurements in low-resolution spectra by avoiding issues related to blends .
In turn , we improve our chemical tagging in such spectra at low metallicities .

Methods : We developed an empirical way of deriving [ Fe/H ] in CEMP ( and C-normal ) stars that relates the equivalent width ( EW ) of strong lines , which remain detectable in lower-resolution , metal-poor spectra , such as X-Shooter spectra to [ Fe/H ] .

Results : The best [ Fe/H ] tracers are found to be Cr I and Ni I , which both show strong transitions in spectral regions that are free of molecular bands ( between \sim 5200 - 6800 Ã , a region accessible to most surveys ) .
We derive different relations for dwarfs and giants .
The relations are valid in the ranges \sim -3 < [ Fe/H ] < -0.5 and 10 < EW < 800 mà  ( Cr ) or [ Fe/H ] > -3.2 and EW > 5 mà  ( Ni ) , depending on the trace element and line as well as the stellar evolutionary stage .

Conclusions : The empirical relations are valid for both CEMP and C-normal stars and have been proven to be accurate tracers in a sample of \sim 400 stars ( mainly giants ) .
The metallicities are accurate to within \pm \sim 0.2 dex depending on the sample and resolution , and the empirical relations are robust to within 0.05-0.1 dex .
Our relations will improve the metallicity determination in future surveys , which will encounter a large number of CEMP stars , and will greatly speed up the process of determining [ Fe/H ] as the EWs only need to be measured in two or three lines in relatively clean regions compared to dealing with numerous blended Fe lines .