Polar surface area (PSA)

Combined surface area over all polar atoms or molecules, primarily oxygen and nitrogen; including their attached hydrogen atoms.

PSA is a commonly used medicinal chemistry metric. Molecules with a polar surface area of greater than 140 angstroms squared are usually believed to be poor at permeating cell membranes. For molecules to penetrate the blood-brain barrier, PSA should be less than 60 angstroms squared.

Fast calculation of molecular polar surface area (Journal of Medicinal Chemistry, 2000)

Quantum mechanical polar surface area (Journal of Computer-Aided Molecular Design, 2012)