SPEE Monograph 4 establishes recommended best practices for decline curve analysis of unconventional wells through the use of specialized diagnostic plots such as the square-root of time plot. These diagnostic plots are important for flow regime identification and selection of an appropriate b-factor for predicting a well’s EUR. However, in practice, because of familiarity with the Arps decline equation and its ease of use, typically a single b-factor is selected despite multi-fractured horizontal wellbores demonstrating changes in flow regime and consequently the b-factor. This paper introduces the Modified Arps Equation, an iterative formulation of the Arps equation. The Modified Arps Equation maintains use of the form of the original Arps equation with a few modifications to allow for a changing b-factor. These modifications allow a single equation to be used with a vectorized b-factor in order to model a well as it transitions from linear flow to boundary dominated flow and eventually back to pseudo-linear flow. This equation may be applied broadly across unconventionals where wells transition over years from linear flow to boundary dominated flow and later return to pseudo-linear flow as simulation data suggest.