Methods of calculating a maintainability index are provided. The methods include, for each function in software code for an existing software product, locating errors related to the function in the software code in a common historical error database; filtering the located errors for the function based on developer experience; calculating a percentage of errors for the function based on the filtered located errors for the function and successful compiles for the function; and providing a maintainability index for the function based on the calculated percentage of errors.