Geotechnical parameters required for estimating the maximum allowable annular pressure (Pmax) in horizontal directional drilling (HDD) are often empirically determined based on the results from the standard penetration test (SPT). Despite the benefits of SPT, its use in practice could be challenging as the geotechnical parameters determined based on SPT results vary largely depending on the choice made for SPT correlations. Such variability within the geotechnical parameters could result in a high degree of uncertainty within the estimates of Pmax in HDD. In this paper, an SPT-based algorithm is developed to study the variation in estimates of geotechnical parameters and Pmax due to the choice made for SPT correlation. Sensitivity analysis conducted using the SPT-based algorithm presents significant variations in both estimates of geotechnical parameters (relative ranges vary from 11% to 1516%) and Pmax (relative ranges vary from 10% to 1313%) for cases where the understanding of soil behavior and characteristics is insufficient. Error analysis conducted based on the errors in the geotechnical parameters determined using SPT correlations and the corresponding estimates of Pmax presents that the two types of errors are found closely related for most cases; however, exceptions also exist for the cases with the SPT correlations related to modulus of elasticity. Based on the results of the error analysis, suggestions for the use of SPT correlations are made for determining the input geotechnical parameters required for estimating the Pmax in HDD, and they are applied to a case study related to the hydraulic fracture experiments to demonstrate their usefulness in a real-world example. The case study presents high precision (R2 > 0.81) in the estimates of Pmax obtained by following the suggestions for the use of SPT correlations. In remark, the estimates of Pmax made based on some SPT correlations are found nearly equal to the measurements of annular pressure at hydraulic fracture, confirming that the SPT-based algorithm and the suggestions for the use of SPT correlations provided herein are reasonably reliable for estimating the Pmax in HDD. These results highlight the challenges with using empirically determined geotechnical parameters in the design of HDD and provide practitioners with valuable information for evaluating the stability of the HDD borehole.