SUMMARYA partial di erential equation to compute the distance from a surface is derived and solved numerically. The beneÿt of such a formulation especially in combination with turbulence models is shown. The details of the formulation as well as several examples demonstrating the in uence of its parameters are presented. The proposed formulation has computational advantages and can be favourably incorporated into one-and two-equation turbulence models like e.g. the Spalart-Allmaras, the Secundov or Menter's SST model.