The measurement uncertainty attributed to gas energy measurements includes, among others, a contribution for the calculation of the average/total from time-sampled data. The aim of this study is to establish and verify a procedure for the evaluation of this component of uncertainty, which considers contributions related to thedeterministic and random variations of the measured data. The first step of the proposed procedure is a separation of deterministic and random components via time-domain filtering, followed by the evaluation of the uncertainty associated to the deterministic component by means of a decimation method and of the uncertaintyassociated to the random component by statistical analysis. The procedure concludes with the calculation of the combined uncertainty due to these effects. Basic guidelines for the optimal selection of procedure parameters related to filtering and decimation are discussed. Demonstration and verification of the proposed procedure on experimental time-sampled data for the superior calorific value and on artificially generated data confirms that the proposed procedure enables reliable and robust evaluation of the uncertainty.