puml.online Tips: Draw Sequence Diagrams, Class Diagrams and More — Instantly

What is puml.online

puml.online is a free online diagram tool that bundles PlantUML, Mermaid, and Excalidraw in one place. Its key advantages:

• Open and start — no sign-up, no download, no second page to navigate
• 100+ templates — can’t remember the syntax? Pick a template and modify it
• Built-in themes — switch skins with one click, no skin code required

Open and Start

Visit https://puml.online. The editor loads instantly — write on the left, see the rendered diagram on the right in real time.

If you need more room, click the Maximize Editor button on the toolbar to fill the browser window.

Choose a Template

Click Templates to open a dialog with ready-made templates covering sequence diagrams, class diagrams, use case diagrams, component diagrams, and more. Selecting one fills the editor with working code — just change the text.

Note: picking a template replaces the current editor content. Save your work first, or copy it somewhere safe before selecting.

Switch Themes

Click Theme to browse and apply built-in PlantUML themes. Dozens of styles are available — switch the look of your diagram without writing any skin code.

Switch Between Mermaid and Excalidraw

Three tabs sit at the top of the editor: PlantUML / Mermaid / Excalidraw. Click any tab to switch the rendering engine. The same diagram can be drawn in different languages without leaving the page.

Privacy and Security

• Mermaid and Excalidraw run entirely in your browser — code never leaves your device
• PlantUML uses a lightweight backend server (because the pure-JS PlantUML build is incomplete), but it stores nothing — it only renders your diagram on the fly

Summary

puml.online is a true “open and start” online diagram editor: no registration, no install, three diagram languages in one place. With templates and themes, it dramatically speeds up diagram creation for engineers who want to get things done.