Understanding the gas compressibility factor is necessary to solve petroleum engineering problems—such as predicting reserve, gas-pressure gradients, gas compression, gas metering, etc. Typically, the gas compressibility factor is obtained by laboratory experiments. Occasionally when laboratory reports are not available, equations of state (EOS) or empirical correlations are used to estimate this important factor. The first step is to determine the pseudo-critical gas properties. This is done by understanding the gas composition and correlations from Lee-Kesler. When the gas composition is unknown, gas properties are determined using correlations based on specific gravity. After obtaining the pseudo-critical data, the compressibility factor can be determined by using the Standing-Katz (SK) Z-factor chart or Dranchuk and Abou-Kassem (DAK) method.