In order to facilitate surface concentration estimation for arsenic diffusion, a simple expression has been derived which is dependent only upon the junction depth xJ and the sheet resistance Rs: C s = 1.56 × 1017/xJRs. This equation was derived from experimental mobility data and a polynomial approximation to the solution of the arsenic diffusion equation. Available data on diffusions carried out from five different kinds of arsenic sources indicate that this derived equation for Cs is accurate to within 15% for Cs≳ni. © 1972 The American Institute of Physics.