OnUML Mermaid Editor

OnUML is a free, browser-based AI diagram generator that combines a Mermaid editor with PlantUML and Draw.io modes, letting you create, edit, and export UML diagrams from text prompts or images.

What is OnUML?

OnUML is an online tool that unifies three diagramming syntaxes in one workspace: Mermaid, PlantUML, and Draw.io (XML). It accepts natural-language text or image uploads as input, converts them into editable diagram code, renders the result in real time, and provides export to PNG or SVG plus shareable links. The platform is free to use for core editing, and you can start without registering an account.

Key Features

• AI-powered diagram generation — Turn natural-language descriptions or uploaded images into editable Mermaid, PlantUML, or Draw.io code.
• Multi-diagram mode support — Switch between Mermaid, PlantUML, and Draw.io editors in the same workspace and see live previews.
• Code version history — Save and restore earlier versions of your diagram code to prevent data loss.
• One-click code repair — Automatically detect and fix syntax errors in PlantUML or Mermaid code with a single click.
• Project-based organization — Save diagrams into projects and workspaces for easier management (limits apply by plan).
• Export and share — Export diagrams as PNG or SVG, or share them via shareable links.
• Professional mind maps — Create structured mind maps with a dedicated diagram type, including direct node editing on the canvas.

Who is it for?

• Software developers — Generate PlantUML class diagrams, sequence diagrams, and use-case diagrams from text descriptions without writing code by hand.
• Technical writers — Embed Mermaid flowcharts, state diagrams, or Gantt charts directly into Markdown documentation using the Mermaid live editor.
• System architects — Use Draw.io mode for architecture diagrams, network layouts, and whiteboard-style planning with drag-and-drop editing.
• Project managers — Create Gantt charts or mind maps to visualize schedules, dependencies, and brainstorming sessions.

What can you do with OnUML?

• Design formal UML models: Build accurate class, sequence, component, and C4 diagrams with PlantUML syntax and AI assistance.
• Document with Mermaid: Produce lightweight flowcharts, sequence diagrams, and Gantt charts that live inside Markdown files.
• Plan visually: Use Draw.io's drag-and-drop canvas for architecture diagrams, org charts, and network diagrams with no coding.
• Repair broken code: Paste invalid PlantUML or Mermaid code into the editor and use the one-click repair feature to fix syntax errors.

How does OnUML work?

1. Choose Mermaid, PlantUML, or Draw.io mode.
2. Describe your diagram in natural language, start from a template, or upload an image.
3. Edit the generated code or manipulate the visual canvas; the preview updates in real time.
4. Export as PNG/SVG or share via a link.

Pricing

OnUML is freemium. The free plan includes core editor access, 2 diagrams per project, 10 cloud projects, 2 workspaces, and 10 free model requests per day. Pro Monthly ($8/month) and Pro Yearly ($79/year) add image upload, unlimited projects and workspaces, 500 monthly credits, and no daily free-model limit. Optional one-time credit packs are available: Starter ($4.90 for 240 credits), Popular ($9.90 for 500 credits), and Pro Pack ($19.90 for 1,100 credits). Credits expire six months after purchase.

FAQ

Is OnUML free?

Yes, OnUML's core features are free and sufficient for regular day-to-day use. Advanced features such as image upload, larger project capacity, and higher usage limits require a paid Pro plan or AI credit packs.

What diagram types does OnUML support?

OnUML supports Mermaid diagrams (flowchart, sequence, Gantt, state, basic ER), PlantUML diagrams (class, sequence, use case, component, deployment, C4), and Draw.io diagrams (architecture, flowchart, UML, org chart, network, whiteboard). It also includes a professional mind map type.

How can I save my diagrams?

You can export your diagrams as PNG or SVG files at any time. With a free or paid account, you can also save diagrams by project in the cloud, manage multiple versions, and restore earlier code versions.

What's the difference between Mermaid, PlantUML, and Draw.io modes?

Mermaid is a lightweight text DSL ideal for quick documentation diagrams. PlantUML supports more formal UML and technical modeling with detailed syntax. Draw.io offers a visual drag-and-drop canvas with the highest layout freedom, requiring no coding.

Are there usage limits?

Yes. The free plan includes 10 free model requests per day and 2 diagrams per project. Pro plans remove the daily free-model request limit and increase diagram/project/workspace capacity. AI credit packs are used for extra generation, image understanding, and premium model runs.