Two Monte-Carlo-based methods for computing the dominance ratio in reactor calculations are the Fission Matrix Method (FMM) and the Coarse Mesh Projection Method (CMPM). The FMM allows an estimate of the dominance ratio to be computed before the fission source has converged, but requires a fine mesh—and hence considerable computational resources—for sufficient accuracy. Conversely, the CMPM gives very accurate results on a coarse mesh with very little computational effort, but can be used only after the fission source has converged. In this paper we describe a new method called the Noise Propagation Matrix Method (NPMM) that has the same coarse-mesh accuracy properties as the CMPM while also permitting an ‘on-the-fly’ estimation of the dominance ratio during fission source convergence. Like the CMPM, the NPMM uses the noise propagation matrix (NPM) in determining the dominance ratio. The two methods differ, however, in how the matrix is used to obtain the dominance ratio. A new derivation of the equations used to compute the NPM in a Monte Carlo calculation is presented that eliminates an approximation made in an earlier work on the CMPM. It is shown that by using the improved expression for the NPM, the dominance ratio can be found directly and simply as the largest-modulus eigenvalue of the matrix—thereby eliminating the more complicated time-series-analysis method used by the CMPM. Results for several problems are presented that demonstrate the validity of the method.