In this work, the focus is on an alternative solution for expressing the uncertainty of a measurement, which is applied in location by the identification through radio-frequency (RFID). A proposal of a model for the expression of measurement uncertainty is presented, named as the maximum possible differential uncertainty (IMPD, in Portuguese), and its application to the Cell-Id (Cid), nearest neighbor (VMP, in Portuguese) and trilateration (3L) methods that may be of use for choosing the relevant parameters for location before implementation. A modification of Cell-ID is proposed, named mean Cell-ID (MCid), and built the error model for both Cid and MCid, taking into account the parameters not considered in the literature. The models proposed in this work conducted, naturally, to the definition of an optimal range for the tags. Consequently, it is derived closed formulae for its direct and fast determination. Finally, with all the tools built, a numerical experiment applied to the location of emergency calls is developed, according to criteria for the E911 (Enhanced 911). The results indicated the possibility of positioning, even if the range shows variations of 40 m, in magnitude, in relation to the spherical range generally adopted in the literature. It was shown how to model the uncertainty of VMP and 3L, and performed a comparison between the measurement uncertainty as given by the method proposed in this work with the one determined by a Monte Carlo simulation. The results indicate that the proposed approach can estimate the measurement uncertainty for high coverage probabilities, and that it is especially useful when one or more input quantities cannot be measured.