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Mermaid vs PlantUML: Which Diagrams-as-Code Tool Should You Use?

Mermaid and PlantUML are the two most popular diagrams-as-code languages. Both turn text into diagrams, but they differ in how they render, where they're supported and how much UML depth they offer.

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At a glance

MermaidPlantUML
RenderingJavaScript in the browserJava server (or hosted server)
Native GitHub/GitLab renderingYesGitLab with config; not GitHub
Syntax styleLightweight, Markdown-likeVerbose, UML-oriented
UML coverageCommon UML diagramsVery broad UML coverage
Charts (pie, XY, sankey, quadrant)YesLimited
Learning curveLowMedium

The same diagram in both

mermaid
%% Mermaid
sequenceDiagram
    Alice->>Bob: Hello
    Bob-->>Alice: Hi!
text
' PlantUML
@startuml
Alice -> Bob: Hello
Bob --> Alice: Hi!
@enduml

When to choose Mermaid

Choose Mermaid when diagrams live in Markdown (READMEs, wikis, Notion, Obsidian), when you want zero infrastructure, and when you also need charts and business diagrams. It's the default for most modern docs.

When to choose PlantUML

Choose PlantUML when you need deep UML features (deployment, object, timing diagrams with many options) or when your organisation already runs a PlantUML server and has existing diagrams.

Frequently asked questions

Can I convert PlantUML to Mermaid?

Many diagrams translate directly. Paste PlantUML into MermaidViewer's AI generator and ask it to convert to Mermaid; review the output for any unsupported features.

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