<p>Predicting the jet grouting column diameter is a fundamental stage in the design process of soil improvement projects. Despite the progressive use of the Simplified method as a straightforward approach for estimating jet column diameter by relying on just three variables of mix design, pump pressure, and energetic efficiency (affected by the in-situ soil characteristics), the literature lacks more in-depth investigations on the input parameters in this method. Furthermore, the formulation of this method considers the pressure lost during the injection path from the pump to the nozzles, which is an unclear parameter for design engineers. To this aim, the accuracy of the Simplified method predictions has been separately addressed for single- and double-fluid injection systems in this study based on the field results of a jet project in silty soil. A detailed calculation example of this method has been presented based on the mix design properties. The results delineated that the upper-bound predictions of the Simplified method were more fitted to measured values for double-fluid systems, while the average values of this method were more suited to the single-fluid mix designs. The range of the energetic efficiency parameter for double-fluid systems was about three times the values for the single-fluid systems. Moreover, it has been shown that using the current formulation of the Simplified method can be extremely overpredicted and thus unconservative, which can cause profound repercussions. Hence, the formulation of this method has been revised based on the pressure at the nozzle, not the pump pressure.</p>