What Is Mermaid Viewer?
Mermaid is a popular open-source diagramming language that lets you define charts and diagrams using a simple, Markdown-like text syntax. ToolRunner's Mermaid Viewer renders your Mermaid code into beautiful SVG diagrams in real time — entirely inside your browser, with no server round-trips.
Related Tools
Supported Diagram Types
- Flowcharts — model processes, decision trees, and workflows
- Sequence Diagrams — visualize interactions between services or components
- Class Diagrams — map out object-oriented class hierarchies and relationships
- State Diagrams — describe state machines and transition logic
- Entity-Relationship Diagrams — design database schemas
- Gantt Charts — plan project timelines and milestones
- Pie Charts, Git Graphs, and more as supported by the Mermaid library
Key Features
- Live preview that updates as you type with a short debounce for performance
- Syntax error detection with clear, descriptive error messages
- Automatic dark-mode support — diagrams re-render when you toggle themes
- Zoom in, zoom out, and reset controls for detailed inspection
- Export diagrams as SVG or high-resolution PNG with one click
- Copy the raw SVG markup to your clipboard for embedding in docs or slides
- Upload .mmd or .mermaid files directly from your file system
- Built-in sample diagrams so you can start experimenting immediately
How to Use
Type or paste Mermaid syntax into the left editor. The diagram renders instantly on the right. Choose a sample from the Sample dropdown to explore different diagram types. Use the zoom controls to inspect details, and click SVG or PNG to download the rendered output. Drag the center divider to resize panes, or click Full to go fullscreen.
Why Use a Browser-Based Mermaid Viewer?
Many Mermaid rendering tools require a Node.js build step or send your diagram code to a remote API. ToolRunner does everything client-side — your proprietary architecture diagrams, internal sequence flows, and database schemas never leave your machine. This is especially valuable when working with confidential system designs.
Common Use Cases
- Drafting architecture diagrams for technical design documents
- Creating sequence diagrams for API interaction documentation
- Building flowcharts for onboarding guides or runbooks
- Generating class diagrams from design discussions
- Exporting diagrams for presentations, wikis, or README files
Frequently Asked Questions
- Is my diagram code sent to any server?
- No. Mermaid rendering happens entirely in your browser via the official Mermaid.js library. Your code stays on your device.
- What Mermaid version is used?
- The viewer uses the latest stable release of the Mermaid library bundled with the app. It supports all standard diagram types included in that version.
- Can I use this tool offline?
- Yes. After the page loads, the Mermaid Viewer works without an internet connection since all rendering is performed client-side.
- How do I fix a syntax error in my diagram?
- When the Mermaid parser detects invalid syntax, a red error banner appears at the bottom of the editor with a description of the issue. Review the indicated syntax and correct it — the preview will update automatically.
- What is the difference between SVG and PNG export?
- SVG export produces a scalable vector file that stays crisp at any size — ideal for documentation and web embedding. PNG export rasterizes the diagram at 2x resolution for use in presentations or image editors.