How can I quantify the amount of technical debt that exists in a project?
If I wanted to help a customer understand the degree of technical debt in his application, what would be the best metric to use? I've stumbled across Erik Doernenburg's code toxicity, and also Sonar's technical debt plugin, but was wondering what others exist.
Ideally, I'd like to say "system A has a score of 100 whereas system B has a score of 50, so system A will most likely be more difficult to maintain than system B".
Obviously, I understand that boiling down a complex concepts like "technical debt" or "maintainability" into a single number might be misleading or inaccurate (in some cases), however I need a simple way to convey the to a customer (who is not hands-on in the code) roughly how much technical debt is built into their system (relative to other systems), for the goal of building a case for refactoring/unit tests/etc.
Again, I'm looking for one single number/graph/visualization, and not a comprehensive list of violations (e.g. CheckStyle, PMD, etc.).