PlantUML vs Mermaid vs Draw.io — which one to reach for

There’s no “best” diagram tool, only the right one for each situation. This is the distillation of several tool switches.

One-line conclusion

Where PlantUML shines

Sequence diagrams are PlantUML’s masterwork. alt / else / opt / loop / par / critical give you proper control-flow semantics:

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@startuml
participant FE
participant API
FE -> API: POST /login
alt success
  API --> FE: 200
else bad credentials
  API --> FE: 401
end
@enduml

Class diagrams too. interface / enum / abstract all express directly:

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@startuml
abstract class Animal {
  +name: String
  {abstract} +makeSound()
}
class Dog {
  +bark()
}
class Cat {
  +meow()
}
Animal <|-- Dog
Animal <|-- Cat
@enduml

And component, deployment, use case, object, state, sequence… PlantUML covers the most of the UML standard. When the team needs strict UML, PlantUML is the answer.

Where Mermaid shines

Integration is the killer feature. GitHub, GitLab, Notion, Obsidian, Hexo, Confluence, Slack — they all parse Mermaid blocks directly:

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flowchart LR
  A[User] --> B{Logged in?}
  B -->|yes| C[Home]
  B -->|no| D[Login]
  D --> B

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erDiagram
  USER ||--o{ ORDER : places
  USER {
    int id PK
    string email UK
    string name
  }
  ORDER {
    int id PK
    int user_id FK
    date created_at
  }

Drop a Mermaid block into a PR description and GitHub renders it for free.

Where Draw.io shines

Draw.io is the final fallback. When PlantUML / Mermaid auto-layout won’t cut it, when the diagram is huge and needs to be split visually, when icons need to be mixed in — open Draw.io.

Hard advantages:

• WYSIWYG, full drag-and-drop
• Layout algorithms selectable (tree, organic, circular)
• Total control over fonts, colors, shadows
• Files are XML, can go in git (though humans can’t diff them)
• Inline SVG / PNG / base64 images supported

But it’s not text-first. Every edit needs the desktop or web UI. There’s no PR diff. Diagrams rot quietly.

Comparison table

My day-to-day recipe

• Code comments / PR descriptions / doc snippets → Mermaid (renders anywhere)
• Sequence, class, state, component, use case → PlantUML
• “Beautiful” architecture diagrams for exec or customer → Draw.io
• One diagram that needs both worlds → Draw.io as canvas, PlantUML/Mermaid as source via plugins

Don’ts

• Don’t cram custom SVG into PlantUML. The auto-layout will scramble your handcrafted shapes — they’re painful to maintain.
• Don’t use Mermaid as your only flowchart tool. Git-friendliness and integration are its strength; switch tools when pixel layout becomes important.
• Don’t maintain reviewable diagrams in Draw.io alone. No diff, no per-edit accountability. Either convert to PlantUML/Mermaid, or accept that git history will only show whole-XML blobs.