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We present uniformly measured stellar metallicities for 463 stars in 13 MilkyWay (MW) ultra-faint dwarf galaxies (UFDs; $M_V = -7.1$ to $-0.8$) usingnarrowband CaHK (F395N) imaging taken with the Hubble Space Telescope (HST).This represents the largest homogeneous set of stellar metallicities in UFDs,increasing the number of metallicities in these 13 galaxies by a factor of 5and doubling the number of metallicities in all known MW UFDs. We provide thefirst well-populated MDFs for all galaxies in this sample, with [Fe/H] rangingfrom -3.0 dex to -2.0 dex, and $\sigma_{\mbox{[Fe/H]}}$ ranging from 0.3 dex to0.7 dex. We find a nearly constant [Fe/H] ~ -2.6 over 3 decades in luminosity(~$10^2 - 10^5 L_{\odot}$), suggesting that the mass-metallicity relationshipdoes not hold for such faint systems. We find a larger fraction (24%) of EMP([Fe/H]<-3.0) stars across our sample compared to the literature (14%), butnote that uncertainties in our most metal-poor measurements make this an upperlimit. We find 19% of stars in our UFD sample to be metal-rich ([Fe/H]>-2.0),consistent with the sum of literature spectroscopic studies. MW UFDs are knownto be predominantly >13 Gyr old, meaning that all stars in our sample are trulyancient, unlike metal-poor stars in the MW, which have a range of possibleages. Our UFD metallicities are not well-matched to known streams in the MW,providing further evidence that known MW substructures are not related to UFDs.We include a catalog of our stars to encourage community follow-up studies,including priority targets for ELT-era observations.

Metallicity Distribution Functions of 13 Ultra-Faint Dwarf Galaxy Candidates from Hubble Space Telescope Narrowband Imaging