You have surely used the Excel inbuilt functions like pmt, sum, average, min, max, etc. Creatig custom functions is easy and provides great flexibility in calculations. You can, for example, use the input of a 'basic salary' to compute the total package that an employee recives based on his different perks. Using the Visual basic editor that comes free with Excel, you can write a simple macro to create a custom function. The training video below describes the complete process.