We report a computationally efficient method for the quantitative prediction of the static dielectric constant (epsilon) of water at high temperatures and pressures. Values of epsilon in this regime are needed in geochemistry for the prediction of ionic solubilities in hydrothermal fluids. The method involves the application of a recently derived perturbation theory to the SPCE model of water. The experimental database for epsilon in this regime is sparse, and the alternative computational procedure-a direct simulation-is 1-4 orders of magnitude slower than the method we present. We find that the calculation provides quantitatively accurate values of epsilon for water provided y less than or equal to 2.4, where y is the "dipolar strength function" (=(4 pi/9)mu(2) rho/kT).