A method of creating a user-defined function in a spreadsheet application is provided. The user-defined function is callable by a calling label in the spreadsheet application. A formula is received in the spreadsheet application. The formula is transferred to a function sheet of the spreadsheet application to define at least a portion of a function body. An interface is defined by identifying the calling label, an input parameter and a result parameter of the user-defined function. The function body operates on an argument corresponding to the input parameter and generates a result corresponding to the result parameter. The interface is stored in association with the function body to create the user-defined function.