We present a theoretical calibration of a new metallicity diagnostic based on the Strömgren index m _ { 1 } and on visual – near-infrared ( NIR ) colors to estimate the global metal abundance of cluster and field dwarf stars . To perform the metallicity calibration we adopt \alpha -enhanced evolutionary models transformed into the observational plane by using atmosphere models computed adopting the same chemical mixture . We apply the new visual - NIR Metallicity–Index–Color ( MIC ) relations to two different samples of field dwarfs and we find that the difference between photometric estimates and spectroscopic measurements is on average smaller than 0.1 dex , with a dispersion smaller than \sigma = 0.3 dex . We apply the same MIC relations to a metal-poor ( M 92 ) and a metal-rich ( 47 Tuc ) globular cluster . We find a peak of -2.01 \pm 0.08 ( \sigma = 0.30 dex ) and -0.47 \pm 0.01 ( \sigma = 0.42 dex ) , respectively .