The pavement structural number (SN) is used in various applications worldwide. One application is the structural condition index. The structural condition index was developed for the Texas Department of Transportation (DOT) to support maintenance and rehabilitation decisions at the network level and is calculated as the ratio of two SN values (effective SN/required SN). A direct method of determining the SN from falling weight deflectometer (FWD) testing has been used to evaluate the effective SN. This study sought to increase the accuracy of the structural condition index by improving the method of determining the effective SN from FWD deflection data. A new equation was developed by modifying the equation that was previously used, and new coefficients for four different flexible pavement types were used. The new equation was improved by the addition of new variables for considering the depth to a rigid layer and by the use of a large database consisting of hypothetical pavement structures and responses. This effort not only benefits the Texas DOT by allowing increased accuracy in implementing the structural condition index but also helps other agencies improve their evaluation of the SN by FWD deflection at the network level. In this study, only surface-treated pavement and asphalt concrete pavement were evaluated; portland cement concrete pavement was not addressed.