The International Organization for Standardization defines two standards for printing speed, known as images per minute and pages per minute. Either IPM or PPM indicates ISO printing speed, but the standards are not interchangeable. In fact, they are measured in slightly different ways with different results. As an international organization, ISO uses international paper sizes for testing purposes.
To measure IPM, the ISO provides three sets of test documents -- Microsoft Word, Microsoft Excel and Portable Document Format files -- each consisting of four pages. The documents contain a single photograph, along with corporate logos, diagrams and graphs, but the emphasis is on textual rather than graphical information. IPM measures the number of regular A4 pages, measuring 8.3 inches by 11.7 inches, printed in one minute.
PPM also measures the number of A4 pages printed per minute, but the principal difference between IPM and PPM is that IPM is measured at a fixed resolution, or number of dots per inch, whereas PPM is not. The fewer drops of ink a printer needs to transfer to paper the faster its printing speed. This means that the lower the resolution, the faster the PPM. Furthermore, only a single ISO test page needs to be printed to measure PPM.
IPM provides a more accurate method of comparing one printer with another than PPM. Bear in mind, though, that IPM is based on a standard set of test documents, so a printer may behave differently when you use it at home. If the quality of the material you print is higher or lower than that used to calculate the IPM, or the size of the paper you're printing on is larger or smaller than A4, the printing speed will differ from the advertised IPM. Note, too, that printing speed may also be affected by the complexity of the printed page, the percentage coverage and the software that creates the document.
IPM and PPM don't correlate -- no mathematical formula allows you to work out one measurement from the other. Typically, IPM refers to ISO test results, while PPM can refer to ISO test results or to results obtained by other, legacy test methods. Compare like with like when assessing printer speeds. Note that ISO IPM speeds are almost always lower than non-ISO PPM speeds.