Uncertainty methodology based on accuracy extrapolation (UMAE) is outlined. This methodology is suitable for evaluating the uncertainty in the prediction of transient scenarios in nuclear reactors when carried out by thermal-hydraulic system codes. It is based on the extrapolation of the accuracy resulting from a comparison between code results and relevant experimental data obtained in small scale facilities. A simplified diagram of the UMAE is compared with a similar one derived for the code scaling, applicability and uncertainty evaluation (CSAU) previously proposed by the U.S. Nuclear Regulatory Commission. A few results related to the full application of the UMAE to a small-break loss-of-coolant accident in a pressurized water reactor, including core uncovery, are also reported.