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Calculating pesticide residue levels in feed items for terrestrial species requires accounting for the application rate of the pesticide, the frequency and interval of application, the half‐life of the pesticide on the food item, and the residue unit dose. Microsoft Excel™–based applications such as the US Environmental Protection Agency's Terrestrial Residue Exposure model (T‐REX) and Terrestrial Herpetofaunal Exposure Residue Program (T‐HERPS) calculate the residue levels in feed items using a recursive sequence. A recursive sequence is an unwieldy calculation method that presents a barrier to creating a software‐based tool capable of conducting flexible assessments. Therefore, we determined the closed form of the recursive mathematical equation used by both T‐REX and T‐HERPS. With this formula, we can both duplicate screening‐level assessments (T‐REX, T‐HERPS) as well as incrementally refine the assessment with data‐driven inputs. Integr Environ Assess Manag 2018;14:703–709. © 2018 SETAC Key Points: We determined the closed form of the recursive mathematical equation used by both the Terrestrial Residue Exposure model (T‐REX) and the Terrestrial Herpetofaunal Exposure Residue Program (T‐HERPS).With this formula, applications can be created that use data‐driven inputs not currently allowed by T‐REX or T‐HERPS. [ABSTRACT FROM AUTHOR]