Mermaid vs PlantUML in 2026: Which Should You Use?

Mermaid and PlantUML solve the same problem: write a few lines of text, get a diagram, keep the text in version control. They differ in almost every other way. One runs in a browser tab; the other needs a Java runtime. One renders in a GitHub README on its own; the other mostly does not. One has mindmaps and Gantt charts; the other has deployment and timing diagrams.

This comparison is for the moment you pick one for a team or a project. The short answer: Mermaid when the diagram lives next to code or Markdown, PlantUML when you need the UML types Mermaid lacks or very large graphs.

Side by side

Runtime: browser vs Java

Mermaid is a JavaScript library. Anything with a browser engine can render it: a web page, a Markdown preview, an Electron note app, a headless Chromium in CI. There is nothing to install for a reader.

PlantUML is a Java program. You run it locally with a JAR, or you point a plugin at a PlantUML server that does the rendering and returns an image. Most diagram types also call Graphviz for layout, so that is a second install. None of this is hard, but it is infrastructure, and it is why PlantUML rarely renders "for free" inside other tools.

Where each one renders natively

This is the practical difference. A fenced mermaid block renders in GitHub, GitLab, Notion, Obsidian, Azure DevOps wikis, JupyterLab, GitBook, and in Docusaurus or MkDocs with one config line. Write it, commit it, done. The full list is in where Mermaid renders natively.

PlantUML needs a plugin almost everywhere: an IDE plugin, a Confluence macro, a docs-generator extension that calls a server. GitHub does not render it. Teams that use PlantUML typically commit the generated PNG or SVG alongside the source.

Diagram coverage

Both cover the core UML set: sequence, class, state, and entity relationship, plus Gantt and pie. From there they diverge.

PlantUML is the fuller UML implementation. Deployment, component, object, use case, and timing diagrams exist as first-class types. It also has Salt for UI wireframes, nwdiag for networks, Archimate, and renderers for JSON and YAML documents. If your work is formal UML or enterprise architecture, these matter.

Mermaid went the other way, toward diagrams people put in documentation and slides: mindmaps, timelines, user journeys, gitGraph, quadrant charts, sankey, XY charts, block diagrams, and architecture diagrams. None of these have a PlantUML equivalent.

mindmap
  root((Choosing a tool))
    Mermaid
      Browser only
      GitHub renders it
    PlantUML
      More UML types
      Needs Java

Layout quality

PlantUML uses Graphviz, which has decades of work behind it and handles dense graphs well. It also exposes many layout hints: direction per edge, hidden edges to force alignment, together groups. The flip side is that complex PlantUML diagrams often need those hints to look right.

Mermaid uses dagre by default and can use ELK. Results are predictable and need little tuning for diagrams up to a few dozen nodes. Past that, you split the diagram or accept some long edges. The MermanDraw editor adds node and rank spacing sliders, which covers most of the tuning people actually do.

Styling

PlantUML's skinparam system controls nearly every pixel, and there is a theme mechanism on top. Mermaid has themes plus classDef and style lines that work like inline CSS. Mermaid is quicker to make look good; PlantUML is quicker to make look exactly like your corporate template.

AI generation

ChatGPT, Claude, and Copilot write both. Mermaid output is more reliable for two reasons: the grammar per diagram type is smaller, and the training data is full of Mermaid from public READMEs. PlantUML output is solid for sequence and class diagrams and shakier for the long tail of types.

Either way, the workflow is the same: generate, render, paste the error back. With Mermaid that loop runs in a browser tab; with PlantUML you need the renderer set up first. See generating Mermaid with AI for prompt patterns.

The same diagram in both

A small sequence diagram shows how close the two are for shared types. Participants, arrows, and messages line up almost one to one.

sequenceDiagram
    participant B as Browser
    participant A as API
    participant D as Database
    B->>A: POST /orders
    A->>D: INSERT order
    D-->>A: order id
    A-->>B: 201 Created

@startuml
participant Browser as B
participant API as A
database Database as D
B -> A: POST /orders
A -> D: INSERT order
D --> A: order id
A --> B: 201 Created
@enduml

Note the database keyword in the PlantUML version: it draws the participant as a cylinder. Mermaid has no participant shapes in sequence diagrams, one of many small places where PlantUML is more expressive.

Pick Mermaid if

• The diagram lives in a README, wiki, Notion page, or docs site.
• You want zero install for readers and contributors.
• You need mindmaps, timelines, Gantt, gitGraph, or chart types.
• An AI assistant writes most of your diagrams.
• Diagrams stay under a few dozen nodes.

Pick PlantUML if

• You need deployment, component, object, timing, or use case diagrams.
• You produce formal UML or Archimate for architecture reviews.
• Diagrams are large and dense enough that Graphviz layout pays off.
• Your team already runs a PlantUML server or IDE plugins.
• Pixel-level control over appearance is a requirement.

Many teams use both: PlantUML for the architecture document, Mermaid for everything that lives in a repository. The text-based workflow is the same in both, so switching for a specific diagram costs little.

FAQ

Related

• Mermaid cheatsheet — all 19 diagram types on one page.
• Examples gallery — ready-made Mermaid diagrams for architecture, APIs, DevOps, and data.
• Where does Mermaid render natively?