We summarise our on-going project aimed at calibrating the metallicity of M dwarfs. We have selected a large sample of physical binaries composed of an F-, G- or K-dwarf primary and an M-dwarf secondary. High-resolution spectra of the primary components are being analysed in order to determine, in a uniform way, accurate atmospheric parameters, metallicity and abundance of different elements. From low- resolution spectra of the secondary components we derived reliable spectral types and metallicity- dependent spectral indices. Using all this information, we are improving the current spectroscopic and photometric calibrations of M-dwarf metallicity, and testing some new ones. The resulting calibrations will be very useful in the characterisation of the input sample of exoplanet search programs around M dwarfs. In particular, we share some of our targets with CARMENES.