Most people don't know about solve order and possible multiple solves for a given cell on any update. From : http://www.decisionmodels.com/calcsecretsc.htm htt

Most people don't know about solve order and possible multiple solves for a given cell on any update. From : http://www.decisionmodels.com/calcsecretsc.htm http://www.decisionmodels.com/calcsecretsc.htm When a cell in a spreadsheet refers to another cell it must be finally calculated after the cell it refers to. This is called a Dependency. Excel recognizes dependencies by looking at each formula and seeing what cells are referred to. See Dependency Trees for more details of how Excel determines dependencies. Understanding this is important for User Defined Functions because you need to make sure that all the cells the function uses are referred to in the function arguments. Otherwise Excel may not be able to correctly determine when the function needs to calculated, and what its dependencies are, and you may get an unexpected answer. Specifying Application.Volatile or using Ctrl/Alt/F9 will often enable Excel to bypass this problem, but you still need to write your function to handle multiple executions per calculation cycle and uncalculated data.