The aim of this article was to determine the conversion factors for stacked pine and spruce logs with a length of 3–6 m. To this end, we measured stacks and their logs of a total of 3322.12 steres of pine logs and 1468.46 steres of spruce logs. The conversion factors obtained in this work for pine logs of varying lengths negatively correlate with log length. However, statistically significant (p<0.05) differences were only observed between the longest logs as well as logs of 3 and 4 m in length. This may be due to various factors (mainly curvature, the presence of buttresses on butt logs and taper) influencing the stacking process of logs of increasing length. For logs with a length of 3, 4 and 5 m, the average value of the conversion factor increased with the log thickness class, although this dependence was not statistically significant. In the case of spruce raw material, the obtained conversion factors are similar to the values determined in previous studies. There was no significant correlation between the length of the log and its average diameter. A prerequisite for using conversion factors for large-sized logs is accurate stacking, where special attention should be paid to the parallel stacking of individual logs, aligning their front and back sides to be flush and forming the stack so that its top plane is parallel to the ground.