Microstructured reactors most often contain a large number of micrometer-sized, parallel channels, instead of a large undivided reaction volume. Individual microchannels behave as plug-flow reactors without significant axial dispersion and with excellent heat and mass transfer properties. However, since the reaction takes place in a large number of parallel channels, it is important that all channels provide equal residence time and amount of catalyst volume. These issues depend not only on the flow distributor design, but also, for example, on the manufacturing tolerances. Correlations are derived to express the conversion of a multichannel microreactor explicitly as a function of the variance of a number of reactor parameters, viz. the channel flow rate, the channel diameter, the amount of catalyst in a channel, and the channel temperature. It is shown that the influence of flow maldistribution on the overall reactor conversion is relatively small, while the influences of variations in the channel diameter and the amount of catalyst coating are more pronounced. The model outcomes are also compared to experimental results of two microreactors with different catalyst distributions, which show that the presented method is able to provide a quick, though tough estimation of the influence of differences between channels on microreactor performance.