Free Mermaid Diagram Editor with Live Preview

Mermaid Preview: Live Diagram Editor for Technical Documentation

Mermaid Preview is a free online Mermaid diagram editor for developers, architects, product teams, students, and technical writers. Write diagram-as-code in the editor, see the rendered preview immediately, then export the result as SVG or PNG for README files, architecture docs, tickets, design reviews, and presentations.

The goal is simple: make Mermaid diagrams easier to draft before you commit them to Markdown, GitHub, GitLab, Notion, internal wikis, or project documentation. The editor includes starter templates for flowcharts, sequence diagrams, class diagrams, ER diagrams, state diagrams, and Gantt charts so you can begin with a working example instead of a blank file.

Who Uses Mermaid Preview?

• Backend engineers use sequence diagrams to document authentication, payment flows, retries, queues, and API integrations.
• Frontend teams use flowcharts and state diagrams to explain UI states, route guards, onboarding flows, and error handling.
• Architects and platform teams use class diagrams, ER diagrams, and system flowcharts to keep design notes close to source code.
• Technical writers use Mermaid snippets to create diagrams that are easy to review, update, and translate.

Why Diagram-as-Code Helps SEO, Docs, and Teams

Traditional diagram files are hard to review because most changes are hidden inside binary images or proprietary formats. Mermaid diagrams are plain text, which means a pull request can show exactly which service name, arrow label, or decision branch changed. This makes documentation more maintainable and lowers the cost of keeping diagrams accurate.

For public documentation, Mermaid also helps create clear educational content: every example can include the diagram source, the rendered result, and a short explanation. Readers can copy the source, test it in Mermaid Preview, and adapt it to their own project.

Flowcharts

Flowcharts are initiated using the flowchart keyword, followed by orientation codes such as TD (Top-Down), BT (Bottom-Top), LR (Left-to-Right), or RL (Right-to-Left). Join nodes using process shapes and linking paths:

• Standard Rectangle: A[Normal Node]
• Stadium/Pill Shape: A([Rounded pill])
• Diamond Decision: A{Decision Node}
• Circle: A((Circular Node))

flowchart LR
    A[Start Process] --> B{Is Authorized?}
    B -->|Yes| C[Success]
    B -->|No| D[Access Denied]

Sequence Diagrams

Sequence diagrams illustrate dynamic message threads between actors and objects. They are initiated using sequenceDiagram:

• Actors & Participants: Declare with actor Name or participant Name.
• Messages: Use solid arrows (->>) for active requests, and dotted lines (-->) for replies.

sequenceDiagram
    actor Client
    participant API as Gateway Service
    
    Client->>API: POST /resource
    API-->>Client: 200 OK Response

Class Diagrams

Class diagrams visualize object-oriented hierarchy. They declare classes, fields, visibility modifiers (+ for public, - for private), and interface inheritance:

classDiagram
    class Vehicle {
        +String make
        +startEngine() void
    }
    class ElectricCar {
        +batteryPower int
    }
    Vehicle <|-- ElectricCar

Advanced Compilation and Sharing Options

• XML Correction Exporter: Standard Mermaid output may occasionally yield unclosed HTML void tags (like <br>). Our compiler auto-detects and transforms these elements into self-closing XML (<br/>) upon SVG download to guarantee absolute compatibility with third-party software like Sketch, Photoshop, or Inkscape.
• High-Resolution PNG: Click Export PNG to capture a sharp raster graphic rendered at a 2x device-pixel ratio to guarantee crisp lines and clear font displays.
• LZ-String Sharing: Share links are fully compressed utilizing the Lempel-Ziv (LZ) algorithm, reducing code values into a small, url-safe hash payload.

Common Mermaid Preview Workflows

• Draft a README diagram: paste a flowchart, adjust labels until the preview is readable, then copy the Mermaid source into Markdown.
• Debug syntax errors: reduce a broken diagram to the smallest failing snippet, fix brackets or arrows, then rebuild the full diagram.
• Create export assets: render a final diagram, export SVG for crisp documentation, or export PNG for tools that do not support SVG.
• Share a work-in-progress: use the share button to create a compressed URL fragment that can be sent to teammates for review.

Latest Guides & Articles

Want to level up your diagramming skills? Check out our latest tutorials and insights.

Examples, Trust, and Support

Mermaid Preview now includes dedicated pages that explain the project, provide reusable examples, and give visitors a clear way to report issues or request corrections.

• Mermaid Diagram Examples: copy flowchart, sequence, ERD, and state diagram snippets.
• About Mermaid Preview: learn what the site does, who it is for, and how diagram privacy works.
• Contact: report rendering bugs, article corrections, or privacy questions.

Frequently Asked Questions