Scat analyses are commonly applied to study feeding ecology of carnivores. Factors developed in feeding trials are often used to convert dry matter mass of scat remainders to fresh matter mass of killed or scavenged prey. In our study, we aimed at: 1) presenting conversion factors (CFs) of roe deer Capreolus capreolus, European hare Lepus europaeus and house mice Mus musculus digested by wolf Canis lupus, Eurasian lynx Lynx lynx and red fox Vulpes vulpes, 2) comparing CFs derived from fox exposed to different feeding levels (fasted vs non-fasted before each feeding trial), 3) comparing effects of using different mesh sizes in the lab procedure on CFs, 4) comparing effects of applying CFs derived from wolf, lynx and fox to wolf scats, and 5) quantifying biases caused by inappropriate procedures in scat analyses. Feeding level, use of different mesh sizes, and application of predator-specific CFs affected the estimated number of killed prey individuals. The greatest deviations were found for the feeding level in regard to roe deer and European hare, and for the application of lynx CFs of roe deer and European hare to wolf scats. The strong relationship between prey use and CFs in our study may be used to estimate prey numbers from scats more precisely: in cases of low prey use, we suggest applying CFs derived from non-fasted carnivores, and in cases of high prey use CFs from fasted conspecifics are more appropriate. We recommend applying predator-prey specific CFs and using the same mesh size on which these CFs are based. The presented CFs allow recalculation of prey masses and prey numbers from scat analyses, which had been gained by using inappropriate CFs.