Introduction. When developing new dosage forms, one of the important competencies is the analysis of medicinal substances in this form. Instruments are used for analysis. Calibration of a device is the establishment of a relationship between the readings of the device and the size of the measured value. Instrument calibration is closely related to the assessment of the uncertainty of the measurement results. To determine uncertainty (measurement uncertainty) in instrument calibration, a systematic process is used that involves several steps such as data collection, data analysis, and calculation of various types of uncertainty. Methods for this usually use statistical methods. The purpose of this article is to develop a method for calibrating an HPLC instrument, processing measurement results and assessing uncertainty in the chromatographic analysis of drugs using the example of a standard sample of caffeine. Materials and methods of research. The materials and research methods used were a Shimadzu Prominence-i SPD-M3OA HPLC device with a spectrophotometric detector with a diode matrix and a standard sample of caffeine from Chemical Line, batch: SV-210725, valid until 05/31/2026 Results. The calibration process included checking the condition of the device, cleaning it and installing it in working position. To calibrate the device against a standard caffeine sample, the following operations were performed: taking a sample of a standard caffeine sample, dissolving in a volumetric flask, adjusting to an exact volume, and chromatography on an HPLC system. The calculated uncertainty included the sum of uncertainties: uncertainty due to random factors (repeatability factor), uncertainty due to the construction and use of the calibration characteristic, uncertainty in the selection of the volume of water, uncertainty in measuring glassware, uncertainty due to weighing a sample of the substance. Conclusions. As a result of the study, a method was obtained for determining the uncertainty when calibrating a device using the HPLC method using a standard sample of caffeine as an example. An uncertainty budget has been calculated, which does not exceed the uncertainty of the methodology.