In our paper exact formulas are derived for several Key Performance Indicators (KPI’s) for the periodic review inventory system with stochastic demand and ordering in (integer multiples of) fixed lot sizes, also known as the (R, s, nQ)-system. The KPI’s include the fill rate, discrete ready rate, the expected inventory on hand, the expected number of backorders, the expected number of order lines and the expected order size. We will derive general expressions for these KPI’s for both discrete and continuous demand distributions.  For the special case of gamma and normal distributed demand explicit exact expressions are derived  which can easily be implemented in Excel. Based on these results we developed an easy-to-use Excel tool which is used both in bachelor and master level courses. The theoretical results derived in this paper in combination with the user-friendly software tool developed enable students to get exact results for inventory systems which are highly relevant in practice and for which standard textbooks only provide approximations.