RainIDF, a software tool for derivation of rainfall intensity–duration–frequency (IDF) relationship is developed as an Excel add-in by using Visual Basics for Applications (VBA). The tool is integrated with two of the most widely used statistical distributions for determination of IDF relationship: the generalized extreme value (GEV) distribution for annual maxima series, and the generalized Pareto (GPA) distribution for partial duration series (PDS). It provides automated distribution fitting for rainfall data in the form of annual maxima or PDS for multiple intervals, solving and plotting of rainfall IDF curves. RainIDF uses the Solver add-in function in Excel to solve the coefficients of the empirical IDF formula in one step. The methodology built into RainIDF is discussed and rainfall IDF relationships for several stations in Peninsular Malaysia are derived and compared. RainIDF is available for download on GitHub (http://github.com/kbchang/rainidf) as an Excel add-in.