Created attachment 37012 [details] Reproduction file I have created a sample of when the issue occurs. IRR in apache documentation says that covers "20 tries" as shown here https://poi.apache.org/apidocs/dev/org/apache/poi/ss/formula/functions/Irr.html which is identical to Microsoft Excel https://support.office.com/en-us/article/irr-function-64925eaa-9988-495b-b290-3ad0c163c1bc This indicate that the result is the same between the two; however, in the case of apache poi, it seems that the result is returned as NaN (while excel returns the actual result).