Maintainability Index

The Maintainability Index (MI) is a composite score that blends Halstead volume, cyclomatic complexity, and SLOC into a single number representing how maintainable a unit of code is. mehen reports all three commonly used variants.

What mehen emits

Key	Type	Variant
`mi.original`	float	Oman & Hagemeister, 1992 (raw, can be negative).
`mi.visual_studio`	float	Microsoft Visual Studio rescale to 0–100.
`mi.sei`	float	Software Engineering Institute (SEI) variant including comment ratio.

Formulas

Original (Oman & Hagemeister, 1992)

MI_original = 171
            − 5.2 · ln(V_avg)
            − 0.23 · CC_avg
            − 16.2 · ln(LOC_avg)

where V_avg, CC_avg, LOC_avg are averages across the unit.

Visual Studio (rescaled 0–100)

Microsoft rescaled MI to a 0–100 range so a single threshold could be applied across files:

MI_VS = max(0, MI_original · 100 / 171)

SEI (with comment ratio)

The SEI variant adds a documentation term:

MI_SEI = 171
       − 5.2 · log₂(V)
       − 0.23 · CC
       − 16.2 · log₂(SLOC)
       + 50 · sin(sqrt(2.4 · CM))

where CM is the ratio of comment lines to total lines.

How to read it

`mi.visual_studio`	Interpretation (Visual Studio default)
20–100	Maintainable (green).
10–19	Moderately maintainable (yellow).
0–9	Hard to maintain (red).

Visual Studio uses these thresholds out of the box.

MI is a composite — it aggregates a few simpler signals. When MI changes, look at the components (Halstead volume, Cyclomatic complexity, SLOC, CLOC) to understand which input moved. The composite is useful for ranking files and tracking trends; it is not the right tool to argue about a specific function in isolation.

References

Oman, P. & Hagemeister, J. (1992). Metrics for assessing a software system’s maintainability. Proc. Conf. on Software Maintenance. DOI.
Coleman, D., Ash, D., Lowther, B. & Oman, P. (1994). Using metrics to evaluate software system maintainability. Computer 27(8): 44–49. DOI.
Welker, K. D. (2001). The Software Maintainability Index Revisited. CrossTalk — The Journal of Defense Software Engineering, August 2001. PDF (DTIC archive).
Microsoft: Code metrics — Maintainability Index range and meaning.
Radon: Maintainability Index reference implementation.

Overview

Size

Complexity

Function & class shape

Maintainability Index

What mehen emits

Formulas

Original (Oman & Hagemeister, 1992)

Visual Studio (rescaled 0–100)

SEI (with comment ratio)

How to read it

References

See also

​What mehen emits

​Formulas

​Original (Oman & Hagemeister, 1992)

​Visual Studio (rescaled 0–100)

​SEI (with comment ratio)

​How to read it

​References

​See also

What mehen emits

Formulas

Original (Oman & Hagemeister, 1992)

Visual Studio (rescaled 0–100)

SEI (with comment ratio)

How to read it

References

See also