Background: Ultrasound measurement of the inferior vena cava diameter and its respiratory variability are amongst the predictors of fluid volume status. The primary purpose of the present study was to compare the consistency of inferior vena cava diameter measurements and the collapsibility index, obtained with convex and cardiac transducers. A secondary aim was to assess the agreement of the patient’s allocation to one of the two groups: “fluid responder” or “fluid non-responder”, based on inferior vena cava collapsibility index calculation made with two different probes. Methods: 20 experienced clinicians blinded to the purpose of the study analysed forty anonymized digital clips of images obtained during ultrasound examination of 20 patients. For each patient, one digital loop was recorded with a cardiac and the second with a convex probe. The participants were asked to determine the maximal and minimal diameters of the inferior vena cava in all presented films. An independent researcher performed a comparative analysis of the measurements conducted with both probes by all participants. The calculation of the collapsibility index and allocation to “fluid responder” or “fluid non-responder” group was performed at this stage of the study. Results: The comparison of measurements obtained with cardiac and convex probes showed no statistically significant differences in the measurements of the maximal and minimal dimensions and in the collapsibility index. We also noticed that the decision of allocation to the “fluid responder” or “non-responder” group was not probe-dependent. Conclusion: Both transducers can be used interchangeably for the estimation of the studied dimensions.