Amorphous metals are interesting candidates for use as H(2) purification membranes and occur in some applications of H(2) storage. We introduce a general strategy combining density functional theory and statistical mechanics for quantitatively predicting the properties of interstitial H in amorphous metals. We systematically investigate H solubility in amorphous Fe(3)B, comparing our results with ones for a crystalline material with the same composition. H-H interactions in the amorphous material play a crucial role in determining the net solubility. H solubility in the amorphous and crystalline materials differs by orders of magnitude under conditions relevant for practical H(2) purification membranes. Our results give atomic-level insight into the properties of H in amorphous metals that has not been previously available.