Free Online PlantUML Editor & Diagram Generator
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.
- Live PlantUML preview
- Syntax highlighting
- Copy diagram & source
- Download PNG, SVG & .puml
- Built-in templates
- Shareable links
- Privacy-first
- No account required
Focused PlantUML guides
Each one opens this same editor with a relevant starter diagram, alongside a syntax reference for that job.
What is PlantUML?
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.
PlantUML online editor
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.
Features
- Live diagram preview that updates as you type, with a short debounce
- Full code editor with syntax highlighting, line numbers, find & replace, and undo/redo
- Renders locally with the official PlantUML engine - no source is uploaded
- Export to PNG and SVG, or download the .puml source
- Copy the diagram image or SVG markup straight to your clipboard
- Built-in templates for class, sequence, activity, state, component and architecture diagrams
- Zoom, fit-to-screen, pan and fullscreen preview
- Shareable links that encode the diagram source in the URL - no backend involved
- Works offline once loaded, and installs as a PWA
How to create a PlantUML diagram
- 1
Write PlantUML
Type or paste PlantUML between @startuml and @enduml in the editor, or load a template from the Examples menu.
- 2
Preview instantly
The diagram renders locally in the preview pane as you type. Zoom, fit to screen, or go fullscreen to inspect it.
- 3
Fix and refine
If the syntax is invalid, the editor shows a clear error so you can correct it without losing your code.
- 4
Export or share
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.
Supported PlantUML diagram types
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.
PlantUML syntax examples
@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 vs Mermaid
PlantUML 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 |
Why use a private PlantUML editor?
This editor
- Renders locally - your source never leaves the browser
- Works offline once loaded
- No account, no tracking of your diagram content
Server-based editors
- Send your PlantUML source to a remote server to render
- May log or cache the diagrams you create
- Stop working without a network connection
Use cases
- Documenting software architecture and API sequence flows
- Sketching UML class and state diagrams during design reviews
- Embedding diagrams in READMEs, wikis and design docs
- Teaching and learning UML with instant visual feedback
- Producing diagram images for slides without installing Java or Graphviz
Troubleshooting common errors
Nothing renders and no error appears
Why: The diagram is missing its `@startuml` and `@enduml` delimiters, so there is no block for the renderer to process.
Fix: Wrap the source in `@startuml` and `@enduml`. Every diagram type needs them, including the ones with their own start keywords such as `@startmindmap`.
"Syntax error" pointing at a line that looks correct
Why: Usually the previous line - an unclosed quote, a stray brace, or an arrow with the wrong number of dashes. The parser reports where it noticed the problem, not where it began.
Fix: Check the line above the reported one first. Commenting out blocks with `'` at the start of a line quickly isolates the section at fault.
Participant or class names with spaces break the diagram
Why: An unquoted multi-word name is read as several tokens.
Fix: Quote the name and give it an alias: `participant "Order Service" as orders`. Then reference `orders` everywhere else.
The layout puts elements in an unhelpful order
Why: PlantUML delegates positioning to a layout engine, and you do not control coordinates directly.
Fix: Nudge it with `left to right direction`, explicit `together { }` groups, or hidden arrows such as `A -[hidden]-> B` to influence ranking.
A large diagram renders slowly or is cut off
Why: Layout cost grows quickly with the number of elements and relationships.
Fix: Split it into several focused diagrams. A diagram nobody can read at a glance has usually already stopped being useful.
Non-Latin characters or emoji show as boxes in the exported PNG
Why: The rendering font does not include glyphs for those characters.
Fix: Export as SVG instead, which references fonts rather than rasterising them, and renders with whatever the viewer has available.
Limitations
What this editor deliberately does not do, so you know when to reach for something else.
- You describe relationships, not positions - the layout engine decides placement, which you can influence but not fully control.
- Very large diagrams become slow to lay out and hard to read; splitting them is usually the better answer.
- Sprite libraries and icon sets that require network fetches are unavailable, since rendering happens entirely offline.
- PNG export rasterises text with locally available fonts, so unusual scripts may not render; SVG avoids this.
- PlantUML syntax is more capable than Mermaid but has a steeper learning curve, and it is not natively rendered by GitHub or GitLab markdown.
Frequently asked questions
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