We performed the largest and most homogeneous spectroscopic survey of field RR Lyraes (RRLs). We secured≈ 6300 high-resolution (HR, R∼ 35,000) spectra for 143 RRLs (111 fundamental, RRab; 32 first-overtone, RRc). The atmospheric parameters were estimated by using the traditional approach and the iron abundances were measured by using an LTE line analysis. The resulting iron distribution shows a well-defined metal-rich tail approaching solar iron abundance. This suggests that field RRLs experienced a complex chemical enrichment in the early halo formation. We used these data to develop a new calibration of the ΔS method. This diagnostic, based on the equivalent widths of Ca ii K and three Balmer (H δ, γ, β) lines, traces the metallicity of RRLs. For the first time, the new empirical calibration:(i) includes spectra collected over the entire pulsation cycle;(ii) includes RRc variables;(iii) relies on spectroscopic calibrators covering more than three dex in iron abundance; and (iv) provides independent calibrations based on one/two/three Balmer lines. The new calibrations were applied to a data set of both SEGUE-SDSS and degraded HR spectra totalling 6451 low-resolution (R∼ 2000) spectra for 5001 RRLs (3439 RRab, 1562 RRc). This resulted in an iron distribution with a median η=− 1.55±0.01 and σ= 0.51 dex, in good agreement with literature values. We also found that RRc are 0.10 dex more metal-poor than RRab variables, and have a distribution with a smoother metal-poor tail. This finding supports theoretical prescriptions suggesting a steady decrease in the RRc number when moving from metal-poor to metal-rich stellar environments.