There is wide variance since many institutions have adopted institutional definitions for FTE conversion. However, the most widely accepted definition for comparison across institutions is that of the Common Data Set, "a collaborative effort among data providers in the higher education community and publishers as represented by the College Board, Peterson's, and U.S. News & World Report." Integrated Postsecondary Education Data System (IPEDS) data is used for the Common Data Set. In the Common Data Set, full-time equivalent instructional faculty are defined as full-time plus 1/3 part-time. There are also exclusions to faculty who are counted. I have attached a link to the Common Data Set Initiative page, where you can download the Common Data Set in various formats.
Thank lion for your Answer but I still think full time is equal to one (1), I don't understand exactly what you mean by full time plus 1/3 could you elaborate more please
There are better formulas that are more accurate. For example, the U.S. Department of Labor uses (instructional hours * 2.25) + (other hours). However, the (1/3) for part time is the convention used by the Common Data Set so that all institutions are compared equally. Therefore, for part time faculty, you should use (1/3) regardless if the individuals vary. If you are not exact, other institutions will be in similar positions, and your institution may be compared fairly with them.