This technical report presents a method to compute the density@ compressibility factor@ and supercompressibility factor for thermally cracked gas (TCG) for custody transfer using orifice meters. It provides equations@ parameters@ computation flow diagrams@ and example spreadsheet calculations. This technical report applies to TCG mixtures after treatment. See Table 2 for more information on the types of gases covered. It applies for temperature from 90 ??to 120 ??(305 K to 322 K) at pressures up to 300 psig (2 MPa). It is limited to a specific operating region. The method is for the single gas phase only.