Purpose: To quantify the systematic difference between an F-18 FDG automated dispenser (RIID) and an external dose calibrator (DC) in routine dispensing, to evaluate the factors contributing to this difference, and to derive an empirical conversion coefficient and assess its temporal reproducibility. Materials and Methods: A total of 285 routine dispensing records containing instrument measurements without patient information were analyzed. Measurements obtained less than 2 min apart were compared without decay correction, and intervals of 2 min or longer were corrected to the RIID measurement time to obtain the external DC comparison value. Internal consistency was assessed by comparing the chamber vial–derived removed activity with the RIID output, both derived from RIID chamber vial measurements. Validation experiments comprised two fixed-volume decay-tracking runs at 5 and 10 mL performed on different dates with different sources, and a comparison of the same activity measured in a syringe and in a vial identical to the chamber vial on the same external DC. Agreement was assessed using percentage Bland–Altman analysis. For time-split validation, a coefficient derived from the first 8 measurement days (133 records) was applied to the subsequent 9 measurement days (152 records). Results: The RIID output was 0.47 ± 1.29% lower than the input value. In 224 adjacent dispensing pairs, the chamber vial–derived removed activity and the RIID output showed a paired difference of 2.01 ± 19.23 MBq (P = 0.118) and a median relative difference of +0.20% (interquartile range, −0.97 to +1.52%), indicating internal consistency rather than independent accuracy. In routine dispensing, the RIID output was 5.28 ± 1.39% lower than the external DC comparison value (95% confidence interval, −5.45 to −5.12%). In the direct dose calibrator comparison, the RIID internal dose calibrator read 1.58% higher than the external DC; because the two volume conditions were measured on different dates with different sources, an independent volume effect could not be estimated. In the syringe-to-vial comparison, the vial reading was 4.45 ± 1.05% lower than the transferred activity (vial-to-transferred-activity ratio, 0.96 ± 0.01). The empirical conversion coefficient was 0.947 for the full dataset, and applying it to the later dataset yielded a residual relative difference of +0.04 ± 1.40%. Conclusion: The difference in measurement-container geometry between the two measurement systems was the largest contributor among the factors evaluated. The conversion coefficient represents a relationship between the two measurement systems rather than an accuracy correction, and standardization of measurement conditions and traceability-based cross-calibration are required.