This work describes a methodology that allows an accurate correlation for natural gas compressibility factor to be obtained. This methodology was applied to the Parana State natural gas distribution network in Southern Brazil, whose operating conditions were characterized by more than 14,000 pairs of pressure–temperature readings, measured along eight months. Compressibility factors calculated following Peng-Robinson, Patel-Teja-Valderrama, Shell Oil Company, Soave-Redlich-Kwong, and Van der Waals were compared to those given by the American Gas Association equation, which is used by most natural gas companies. Finally, an alternative correlation was proposed, which is highly accurate within the network operating range.