It is envisioned that tritium will be extracted from DCLL blankets using a vacuum permeator. We derive here an analytical solution for the extraction efficiency of a permeator tube, which is a function of only two dimensionless numbers: one that indicates whether radial transport is limited by the PbLi or by the solid membrane, and another that is the ratio of axial and radial transport times in the PbLi.  The permeator efficiency is maximized by decreasing the velocity and tube diameter, and increasing the tube length.  This is true regardless of the mass transport correlation used; we review several and find that they differ little, and the choice of correlation is not a source of significant uncertainty here. The PbLi solubility, on the other hand, is a large source of uncertainty, and we identify upper and lower bounds from the literature data. Under the most optimistic assumptions, we find that a ferritic steel permeator operating at 470 °C will need to be about twenty times larger in volume than previous conceptual designs using niobium and operating at higher temperatures.