For Si wafer production the concentrations of Au and Cu should be below 0.1ppb. The wafers are cut from a 1.2 m long Si single crystal grown by the Czochralski technique from a melt containing initial Au and Cu concentrations of 1ppm each. What portion of the single crystal can be used when the effective segregation coefficients are 0.000025 for Au and 0.00005 for Cu, respectively.