Some newer software versions display the serial number directly on the code entry screen under the input boxes.
There are many third-party websites (like Radio-Code.co.uk or eBay sellers) that charge $10–$20 to look up the code in the Audi database using your serial number. The "Unlock CD/SD" Method: audi rnse code calculator
Most RNS-E calculators operate by cross-referencing the unit's unique with a database of manufacturer codes. Some newer software versions display the serial number
Search the Google Play Store for "RNS-E Code" or "Audi Radio Code." Several free apps exist that claim to calculate the code based on the serial number prefix. Search the Google Play Store for "RNS-E Code"
Because modern RNSE codes are often derived from a database rather than a simple mathematical formula, generic "calculators" often fail. Instead, there are online services (both free databases and paid locksmith services) where you input your serial number. If your serial exists in their decoded database, they provide the code.