What Is Bus Factor in Software Engineering?

Bus factor in software is a measurement of how many team members would need to leave or become unavailable before a project stalls due to lack of knowledge. A bus factor of one means that a single departure could halt progress on a critical system. The term originated as a blunt thought experiment: "What happens if someone gets hit by a bus?" It has since become a standard metric for evaluating knowledge distribution and team resilience in engineering organizations.

A low bus factor signals concentrated risk. When only one developer understands how the deployment pipeline works, or only one engineer can debug the billing integration, the team operates on borrowed time. Any unplanned absence, whether from illness, job change, or a simple vacation, can create a bottleneck that delays releases and frustrates stakeholders.

A 2019 study published in the IEEE Transactions on Software Engineering analyzed over 1,000 open-source projects and found that nearly 65% had a bus factor of one or two for their core modules. Commercial teams face similar exposure but often do not measure it until a departure forces them to confront the gap. The cost of recovery includes not just lost velocity but also the bugs and regressions that appear when someone unfamiliar with a subsystem is forced to make changes under pressure.

Bus factor is closely tied to knowledge silos and tribal knowledge. Silos form when expertise clusters around individuals, and tribal knowledge stays undocumented. Together, these patterns drive the bus factor down and make teams fragile.

Measuring bus factor starts with mapping who knows what. Some teams use a simple spreadsheet that lists each major component or service alongside the developers who can confidently modify it. If any row has only one name, that component has a bus factor of one. More sophisticated approaches use git history analysis to identify files or directories where commits come from a single author.

Raising the bus factor requires deliberate effort. Code reviews are a natural knowledge-spreading mechanism, but only when reviewers genuinely study the changes rather than rubber-stamping them. Pair programming and rotation schedules, where engineers periodically work on unfamiliar parts of the codebase, also help distribute expertise. Documentation plays a role too, especially architecture decision records and runbooks that capture operational procedures.

The goal is not to make every engineer an expert in every system. That is neither practical nor necessary. Instead, the aim is to ensure that at least two or three people can navigate each critical path, reducing the chance that a single point of failure stalls the team.

Git analysis tools like git-fame and truck-factor calculators can surface bus factor metrics from commit history. Project management platforms sometimes offer code ownership views that highlight concentration. Glue helps teams address the underlying problem by making codebase knowledge searchable and shareable, so context is not locked inside any one person's memory. Teams that combine automated analysis with intentional knowledge-sharing practices tend to build the most durable resilience.

For a comprehensive guide on identifying and improving your team's bus factor, read Bus Factor in Software Engineering.

What is a good bus factor for a software team?

A bus factor of three or higher for each critical component is a common target. This means at least three team members can maintain and modify the system independently. Reaching this threshold typically requires a combination of documentation, code review practices, and deliberate rotation.

How do you calculate bus factor from git history?

Analyze commit authorship per file or module. Count the minimum number of distinct contributors whose combined commits cover a significant majority (often 50% or more) of the changes. If removing one author's commits leaves a module with almost no history, the bus factor for that module is one.

Can bus factor be too high?

In theory, a very high bus factor means many people know the system well, which is positive. In practice, an extremely high number of contributors to a single module can indicate unclear ownership, which introduces coordination overhead. The balance is having enough knowledgeable contributors to avoid fragility without diluting accountability.