How Much of Your Codebase Is Actually Reused?
Code reuse ratio measures how much of your codebase runs through shared libraries, utility functions, or shared components rather than being written from scratch. A high ratio means you're leaning on already-tested code instead of reimplementing the same logic repeatedly, which helps both development speed and code quality. Duplicated code — the same or near-identical logic copy-pasted in multiple places — is the opposite story: the line count grows, but so does your maintenance burden, without any real quality gain.
This calculator divides reused lines by total lines to get your reuse ratio, divides duplicated lines by total lines to get your duplication ratio, and reports the remainder as uniquely written code. As a rule of thumb, a reuse ratio above 30% suggests you're leaning well on shared assets, and a duplication ratio above 15% is often a signal that refactoring is overdue — though the right numbers vary by project and domain.
The most reliable way to cut duplication is extracting repeated logic into shared functions, hooks, or components. If the same code is used across multiple projects, splitting it into its own versioned internal package is often worth the effort. Static analysis tools like jscpd or PMD CPD can measure your actual duplicate line count automatically, giving you accurate numbers to plug into this calculator.
Frequently Asked Questions
It varies by project, but reusing 30% or more of your code through shared libraries or utility functions is generally considered a healthy level.
When the same logic is scattered across multiple places, fixing a bug means finding and updating every copy, which raises maintenance cost and the risk of missing a spot.
Extract repeated logic into shared functions, components, or internal libraries, and split code shared across multiple projects into its own package.