In an image-data processing system, as would be found in a digital copier or scanner, a relatively quick calculation is performed to estimate a compression ratio or compression time resulting from applying a compression algorithm to an image data set. If the estimated compression ratio or compression time is determined to be unfavorable, the system is advised not to apply the compression algorithm. The method can also be used to select a type of compression algorithm, such as CCITT G4 or JBIG2, according to the requirements of the larger system.