The objective of this study was to compare uncertainty evaluation methodologies for very small flow rates, close to zero, during calibration. To exemplify the mathematical treatment of the uncertainty evaluation, experimental values arising from a syringe pump calibration were used to assess the viability and advantage of using alternative methods to the traditional GUM approach (law of propagation of uncertainty), namely, Monte Carlo and Bayesian methods, to quantify measurement uncertainty when the conditions for the applicability of the GUM uncertainty framework are not met. Such conditions relate to the extent of the non-linearity of the model, the number and magnitude of quantities with non-normal distributions and the ability to produce meaningful results for a specific application. Validation should always be attempted whenever the application of the GUM is questionable. Files contained in the dataset are: A2.3.4 Microflow.pdf: Report “Uncertainty calculation methodologies in microflow measurements: comparison of GUM, GUM-S1 and Bayesian approach” Case 1.xls: Excel spreadsheet containing the data associated with case study 1. Case 2.xls: Excel spreadsheet containing the data associated with case study 2.