The secondary dendrite arm spacing at a given cooling rate has been calculated during the solidification by usig the numerical method. The Feuer-Wunderlin coarsening parameter valid for the binary alloys has been applied for the ternary AlCuSi system on the basis of the Tensi-Fuchs, Beaverstock and Roosz methods. The solidification time has been substituted from the real cooling curves then the calculated secondary dendrite arm spacing has been compared with the experimental data. It has been stated by the further investigations that the numerical method can be substituted by a much simpler function by which the secondary dendrite arm spacing developing at the end of solidification can exactly be determined.