PlantUML vs Mermaid in Confluence: an honest comparison

PlantUML and Mermaid solve the same problem — diagrams written as text — and both are open-source engines you can run, read, and extend. If you're choosing one for engineering docs that live in Confluence Cloud, the comparison is worth doing properly, because the factor that decides it in other contexts (where the text renders natively) doesn't apply here: Confluence renders neither out of the box.

Two engines, two rendering models

The architectural difference explains most of the practical ones.

Mermaid renders in the browser. It's a JavaScript library; anywhere that can run JS can turn Mermaid text into a diagram, which is why repo hosts render it natively in Markdown files. No server, no round trip.

PlantUML renders on a server. It's a Java engine (using Graphviz for layout of several diagram types); the text is sent to a PlantUML server, which returns an SVG or PNG. That server can be the public one at plantuml.com or one you run yourself from the official Docker image.

In a GitHub or GitLab README this difference is decisive — Mermaid wins by default because the platform renders it and doesn't render PlantUML. In Confluence Cloud, neither is native. Both require a Marketplace app, which resets the comparison to the merits.

Where PlantUML is stronger

• Diagram breadth and depth. Sequence, class, component, state, activity, deployment, timing diagrams, plus a standard library that includes C4 model support. Teams documenting system architecture tend to hit Mermaid's edges sooner than PlantUML's.
• Layout at scale. Graphviz-backed layout handles large, densely connected diagrams better than browser-layout approaches — the point where a big component diagram stops being readable arrives later.
• Practitioner experience backs this up. One published account of moving architecture docs into Confluence describes abandoning Mermaid because "even simple diagrams looked bad by default" and porting a C4 container diagram to PlantUML in 13 lines.
• Preprocessing. Includes, variables, and skin parameters let big teams share styling and diagram fragments — with the caveat that those includes now live somewhere, which matters when you migrate.

Where Mermaid is stronger

• Zero infrastructure. No render server exists in the story at all — nothing to host, nothing to send diagram text to.
• Ubiquity in dev tools. Native rendering in the major repo hosts means your README diagrams and your wiki diagrams can share a syntax if you standardize on Mermaid everywhere.
• Lighter syntax for simple diagrams. For a quick flowchart, Mermaid is typically fewer characters and fewer surprises.

What recent comparisons conclude

Independent write-ups keep converging on the same split: a 2025 comparison of the two for sequence diagrams and a 2026 engineering-docs comparison both land on: Mermaid for repo-level docs, PlantUML for architecture documents that live in Confluence or a wiki. The reasoning matches the architecture above — where rendering is native, use what's native; where it isn't, pick the stronger engine for long-lived architecture diagrams.

The Confluence-specific question: where does your text go?

In Confluence, both engines arrive via an app, so add one comparison axis that README-context articles skip: the data path. A Mermaid app renders in the browser; your diagram text stays in the page. A PlantUML app sends encoded diagram source to a render server — which is either the public server or one you control. If your diagrams describe internal systems, that's not a footnote; it's a requirement to route rendering through your own server. PlantUML supports exactly that (the server is open source; setup guide here), and it's the configuration we'd point any security-conscious team toward regardless of whose app they use.

The honest caveat

If your team already writes Mermaid everywhere, your diagrams are simple, and consistency with your READMEs matters more than diagram range — Mermaid in Confluence via a Mermaid app is a sound choice, and several Marketplace apps support it (some render both engines). This isn't a category with one winner; it's a category with a good default per use case. Ours: architecture documentation that lives and evolves in Confluence is PlantUML's home turf.