Using an approximation of Bjarngard and Petti [Phys. Med. Biol.33, 21–32 (1988)] that the scatter‐to‐primary ratio SPR(r,d) is a linear function of the geometrical parameterz=rd/(r+d) (whereris the radius of photon beam anddis depth in phantom), a method was developed to determine primary dose directly from depth‐dose data, measured within the range of fields with lateral electronic equilibrium. This method was evaluated using tabulated TAR data for Co‐60 γ radiation in water. The results for primary dose agree within ∼1% of those from other studies. It was shown that this method is applicable within the energy range of therapeutic x rays. Primary dose was determined in the case of 6‐ and 18‐MV x rays using proposed method.