Context :

Aims : We present a new calibration of the Calcium II Triplet equivalent widths versus [ Fe/H ] , constructed upon K giant stars in the Galactic bulge .
This calibration will be used to derive iron abundances for the targets of the GIBS survey , and in general it is especially suited for solar and supersolar metallicity giants , typical of external massive galaxies .

Methods : About 150 bulge K giants were observed with the GIRAFFE spectrograph at VLT , both at resolution R \sim 20,000 and at R \sim 6,000 .
In the first case , the spectra allowed us to perform direct determination of Fe abundances from several unblended Fe lines , deriving what we call here high resolution [ Fe/H ] measurements .
The low resolution spectra allowed us to measure equivalent widths of the two strongest lines of the near infrared Calcium II triplet at 8542 and 8662 Å .

Results : By comparing the two measurements we derived a relation between Calcium equivalent widths and [ Fe/H ] that is linear over the metallicity range probed here , -1 < [ Fe/H ] < +0.7 .
By adding a small second order correction , based on literature globular cluster data , we derived the unique calibration equation \mathrm { [ Fe / H ] _ { CaT } } = -3.150 + 0.432 W ^ { \prime } +0.006 W ^ { \prime 2 } , with a rms dispersion of 0.197 dex , valid across the whole metallicity range -2.3 < [ Fe/H ] < +0.7 .

Conclusions :