Monorepo vs Polyrepo Build Calculator

About the Monorepo vs Polyrepo Build Calculator

Choosing between a monorepo and polyrepo (multi-repo) architecture has significant implications for build times, CI/CD costs, and developer workflow. This calculator helps you compare build times under both strategies based on your project's characteristics.

In a monorepo, build systems like Turborepo, Nx, or Bazel can detect which packages are affected by a change and rebuild only those. The trade-off is longer initial setup and a larger repository. In a polyrepo, each repository has its own build pipeline, but changes to shared dependencies may trigger builds across multiple repos.

The optimal choice depends on your team size, number of packages, cross-package dependency frequency, and CI infrastructure. This calculator quantifies the build time difference to inform your decision.

When This Page Helps

Monorepo vs polyrepo debates often lack data. It gives concrete build time comparisons based on your project parameters, helping you make an evidence-based architecture decision.

How to Use the Inputs

1. Enter the total number of packages or services.
2. Enter the average build time per package in minutes.
3. Enter the number of packages affected per typical change (for monorepo).
4. Enter the number of repos triggered per change (for polyrepo).
5. Enter the dependency resolution overhead per repo (for polyrepo).
6. Compare the monorepo and polyrepo build times.

Example Calculation

Tips & Best Practices

• Monorepos shine when packages share many dependencies and change together frequently.
• Polyrepos are simpler when teams are autonomous and packages rarely interact.
• Use Nx or Turborepo for monorepo build optimization with affected-change detection.
• In polyrepos, pin dependency versions to avoid cascading rebuild triggers.
• Benchmark actual build times before migrating — overhead varies by tooling.
• Consider a hybrid approach: monorepo for related packages, separate repos for independent services.

The Build Time Equation

Build time in both architectures depends on three factors: the work to be done, the overhead per build, and the degree of parallelization. Monorepos reduce redundant work through shared caches and affected-change detection. Polyrepos eliminate unrelated work by scoping each build to one repository.

Tooling Makes the Difference

Raw monorepo builds without optimization are slower than polyrepo builds because they attempt to build everything. The entire value proposition depends on intelligent build tools: Nx, Turborepo, Bazel, or Buck. Without these, monorepo build times grow linearly with package count.

Making the Decision

Consider your team's cross-package change frequency, dependency sharing patterns, and CI infrastructure. If more than 30% of changes touch multiple packages, a monorepo with build optimization likely saves time. If packages are truly independent, polyrepo simplicity wins.

Frequently Asked Questions

• When is a monorepo faster?

  Monorepos are faster when: (1) many packages share dependencies, (2) changes frequently affect multiple packages, (3) you use build tools with affected-change detection, and (4) the alternative would trigger multiple polyrepo pipelines for shared changes. Comparing your results against established benchmarks provides valuable context for evaluating whether your figures fall within the expected range.

• When is a polyrepo faster?

  Polyrepos are faster when: (1) packages are independent with few shared dependencies, (2) changes rarely cross package boundaries, (3) teams work autonomously on separate services, and (4) individual repos are small and build quickly. Reviewing these factors periodically ensures your analysis stays current as conditions and requirements evolve over time.

• What is the overhead in monorepo builds?

  Monorepo overhead includes change detection analysis, dependency graph computation, and cache management. With tools like Nx or Turborepo, this is typically 1–3 minutes. Remote caching can further reduce rebuilds by reusing previous outputs.

• What is the dependency overhead in polyrepos?

  Each polyrepo build must resolve its dependencies independently: downloading packages, building shared libraries, and verifying versions. This adds 1–5 minutes per repo. Monorepos avoid this by using shared node_modules or build caches.

• Can I migrate incrementally?

  Yes. Many teams start with a polyrepo and gradually move related packages into a monorepo. Tools like Git subtree or repo-tool help merge histories. The reverse (splitting a monorepo) is harder but possible with git filter-repo.

• How do CI costs differ between approaches?

  Monorepo CI costs are typically lower because affected-change detection skips unmodified packages. Polyrepo CI can be wasteful when shared dependency updates trigger full rebuilds across all consuming repos.