The impact factor was originally designed for evaluating journals, and the H-index was designed to evaluate authors. But if you insist upon measuring the IF of a researcher, you could do it the same as measuring IF of journal, that is, divide the citations the author receives in 2018, by the number of publications of this author during 2016 and 2017.
I have never heard or seen to calculate impact factor for the authors. We have i10 index and also h index to evaluate the researchers publications provided by different indexing websites.
Definition: The g-index was suggested in 2006 by Egghe. With your articles ranked in decreasing order of number of citations, the g-index is the largest number of highly cited articles for which the average number of citations is at least n. In comparison to h-index, g-index gives more weight to highly cited articles.
Access: Download Harzing’s Publish or Perish (http://www.harzing.com/pop.htm). Search for your name, and your g-index will appear at the top of the page. (Reference: http://naepub.com/publishing/publication-metrics/)
Journal impact factor considers only the impact of particular journals. It is likely that one article in the journal might have been highly cited and another article hardly at all, but both the authors are judged equally based on the high impact factor of that journal. To overcome this problem, “author impact factors” have been suggested, which measure the impact of individual authors. The author impact factor is based on the scientific value of a given researcher or author. You can try the h-index, g-index, i10-index, or compile cited references by using Web of Science, Google Scholar, Scopus, Microsoft Academic, or ResearchGate.