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Create, edit, preview, and export PlantUML diagrams directly in your browser. Diagrams render locally with the official PlantUML engine, so your source code stays private and is never uploaded.
PlantUML is an open-source language for creating diagrams from plain text. Instead of dragging shapes around a canvas, you describe a diagram in a concise, readable syntax and let the engine draw it. Because the source is just text, diagrams are easy to version with Git, review in pull requests, and keep in sync with your code and documentation.
PlantUML covers the full range of UML - class, sequence, use case, activity, state, component, deployment, object, package and timing diagrams - as well as entity-relationship diagrams, mind maps, Gantt charts, work breakdown structures and architecture diagrams.
How this editor works - and why it's different.
This is a full PlantUML editor that runs entirely in your browser. The left pane is a real code editor with syntax highlighting, line numbers, find and replace, code folding and undo/redo. The right pane previews your diagram, updating automatically a fraction of a second after you stop typing.
Crucially, the diagram is rendered locally by the official PlantUML engine compiled to JavaScript and served from LocalOnly's own origin - there is no remote PlantUML server. Your diagram source is never uploaded. When you are done, export to PNG or SVG, download the .puml source, copy the image to your clipboard, or create a shareable link that carries the source in the URL.
Type or paste PlantUML between @startuml and @enduml in the editor, or load a template from the Examples menu.
The diagram renders locally in the preview pane as you type. Zoom, fit to screen, or go fullscreen to inspect it.
If the syntax is invalid, the editor shows a clear error so you can correct it without losing your code.
Download PNG, SVG or the .puml source, copy the image to your clipboard, or create a shareable link that carries the source in the URL.
Class diagrams
Classes, attributes, methods and inheritance.
Sequence diagrams
Ordered messages between participants.
Use case diagrams
Actors and the goals they pursue.
Activity diagrams
Flows, decisions and parallel paths.
State diagrams
States and the transitions between them.
Component diagrams
Software components and dependencies.
Deployment diagrams
Nodes, artifacts and runtime topology.
Architecture diagrams
System, microservices and cloud views.
ER diagrams
Entities, attributes and cardinality.
Gantt charts
Project schedules with dependencies.
Mind maps
Branching maps of ideas.
JSON & YAML
Visualize structured data as a tree.
@startuml
Alice -> Bob : Authentication Request
Bob --> Alice : Authentication Response
@enduml@startuml
class User {
+String name
+login()
}
User <|-- Admin
@enduml@startuml
start
:Receive request;
if (authorized?) then (yes)
:Process;
else (no)
:Reject;
endif
stop
@endumlPlantUML and Mermaid are both text-to-diagram tools. They overlap, but suit different jobs.
| Aspect | PlantUML | Mermaid |
|---|---|---|
| Diagram breadth | Very broad - full UML plus many non-UML diagrams | Growing, but narrower set of diagram types |
| Syntax style | Concise, keyword-driven DSL | Lightweight, markdown-friendly |
| Typical rendering | A rendering engine (here, compiled to run in your browser) | A pure JavaScript library |
| Best fit | Detailed software design and architecture diagrams | Quick diagrams embedded in markdown docs |
Pretty-print JSON with configurable indentation and instant validation.
Encode text or files to Base64, with an optional URL-safe variant.
Percent-encode text for safe use in URLs and query strings.
Decode a JWT to inspect its header, payload, and claims instantly.
Looking for more? Browse all LocalOnly tools.