We present a new direct spectroscopic calibration for a fast estimation of the stellar metallicity [Fe/H]. This calibration was computed using a large sample of 451 solar-type stars for which we have precise spectroscopic parameters derived from high quality spectra. The new [Fe/H] calibration is based on weak Fe I lines, which are expected to be less dependent on surface gravity and microturbulence, and require only a pre-determination of the effective temperature. This temperature can be obtained using a previously presented line-ratio calibration. We also present a simple code that uses the calibrations and procedures presented in these works to obtain both the effective temperature and the [Fe/H] estimate. The code, written in C, is freely available for the community and may be used as an extension of the ARES code. We test these calibrations for 582 independent FGK stars. We show that the code can be used as a precise and fast indicator of the spectroscopic temperature and metallicity for dwarf FKG stars with effective temperatures ranging from 4500 K to 6500 K and with [Fe/H] ranging from -0.8 dex to 0.4 dex. Based on observations collected at La Silla Observatory, ESO, Chile, with the HARPS spectrograph at the 3.6-m telescope (072.C-0488(E)).The code is available for download at http://www.astro.up.pt/~sousasag/TMCalc.