We derive an updated, analytic formula for the gravitational radius, rg, of an irradiated, thin disk. For such a disk, the region outside the gravitational radius will produce a thermal wind, while the region interior to the gravitational radius will be stable. We find that rg ~ 1.4 (M*/Msolar)/(T0/104K)AU. The value of 1.4AU is approximately one fifth of currently used values. The analysis uses the adiabatic approximation. We argue that the same formula applies for the non-adiabatic, isothermal case.