Maintainability Index

What is it?

The Maintainability Index (MI) is a composite score (0-100) designed to indicate how maintainable (easy to support and change) the source code is.

It is calculated using a polynomial equation that combines: - Halstead Volume: Measures the size of the implementation (vocabulary and length). - Cyclomatic Complexity: Measures the control flow complexity. - Lines of Code: Measures the physical size.

How to read it?

It gives you a single number to judge a file's health at a glance.

Score	Rating	Meaning
85-100	🟢 A	Excellent. Easy to maintain.
65-84	🟡 B	Good. Moderate maintainability.
< 65	🔴 C	Bad. Hard to maintain. Consider refactoring.

Limitations

Context matters

A complex algorithm (like a parser or a mathematical computation) might naturally have a lower score. But for standard business logic, controllers, or services, you should aim for green.

The Maintainability Index is best used as a trend metric. If it drops over time, your technical debt is increasing.