Maintainability Index

What It Measures

This metric combines Halstead Volume (information content and program size), Cyclomatic Complexity (control flow paths), and Lines of Code (physical size) into a single 0-100 score. Higher scores indicate more maintainable code.

The Formula

Stellarion uses the standard Microsoft-normalized formula:

MI = MAX(0, (171 - 5.2×ln(HV) - 0.23×CC - 16.2×ln(LOC)) × 100/171)

Components:

• HV = Halstead Volume (information content)
• CC = Cyclomatic Complexity (control flow paths)
• LOC = Lines of Code (physical size)
• ln = Natural logarithm

The formula normalizes to 0-100, with Halstead Volume weighted at 5.2×ln(HV), Cyclomatic Complexity at 0.23×CC, and Lines of Code at 16.2×ln(LOC). The physical size (LOC component) has the strongest influence on the final score.

Rating System

Stellarion classifies maintainability using these thresholds:

Why It Matters

This metric serves as a leading indicator for:

Development Velocity: Code with low MI slows development. Teams spend more time understanding, debugging, and carefully modifying complex code.

Technical Debt: MI below 50 indicates accumulating technical debt. Each change becomes riskier and more expensive.

Onboarding Efficiency: New team members struggle with low-MI codebases. High MI correlates with faster onboarding and knowledge transfer.

Defect Probability: Studies show strong correlation between low MI and defect density. Poor maintainability leads to more bugs.

How to Improve Your Score

Since MI is a composite metric, improvements target its components:

1. Reduce Cyclomatic Complexity (Primary Driver)

The CC component has the most direct impact:

• Extract complex logic into smaller functions
• Use guard clauses and early returns
• Replace conditionals with polymorphism or lookup tables

2. Decrease Lines of Code

Reduce function and file length:

• Split large functions into focused units
• Extract classes and modules by responsibility
• Remove dead code and redundant logic

3. Simplify Halstead Volume

Reduce operator and operand variety:

• Use consistent naming conventions
• Simplify expressions
• Avoid deep nesting that requires mental tracking

Improvement Targets

Stellarion recommends these refactoring targets based on current MI:

Practical Example

Consider a file with:

• Halstead Volume: 2000
• Cyclomatic Complexity: 25
• Lines of Code: 400

MI = MAX(0, (171 - 5.2×ln(2000) - 0.23×25 - 16.2×ln(400)) × 100/171)
MI = MAX(0, (171 - 39.5 - 5.75 - 97.1) × 0.585)
MI = MAX(0, 28.65 × 0.585)
MI = 16.8 (Poor)

After refactoring to reduce CC to 10 and splitting into smaller files (150 LOC each):

MI = MAX(0, (171 - 39.5 - 2.3 - 81.2) × 0.585)
MI = 48 × 0.585 = 28.1 (Fair)

Further improvements to reduce Halstead Volume would push MI into the Good range.

For the complete metrics reference, see Stellarion Quality Metrics.