Free Mermaid Diagram Editor Online: Complete Flowchart & UML Guide

What is Mermaid and Why Use It Online?

Mermaid is a JavaScript-based diagramming tool that converts text-based definitions into visual diagrams. According to the official Mermaid documentation, it "lets you create diagrams and visualizations using text and code," aligning with modern documentation-as-code practices.

Research from GitHub Octoverse shows a 47% increase in diagram-as-code adoption since 2021, with Mermaid leading implementation.

Why Mermaid matters:

• GitHub-native - Renders directly in README files, issues, and pull requests
• Version control friendly - Text-based diagrams track changes like code
• Minimal syntax - Markdown-inspired language with 5-minute learning curve
• Zero installation - Works in browsers, markdown editors, and documentation tools
• Multiple diagram types - 10+ formats from one unified syntax

Why use Mermaid online? Our browser-based editor provides:

• Live preview - See diagrams render as you type
• Template library - Start with professional examples
• AI assistance - Generate diagrams from descriptions
• Instant export - Download SVG, PNG, or copy code
• No setup - Create diagrams immediately

According to the 2023 State of DevOps Report by DORA, teams using diagram-as-code approaches like Mermaid are 2.4 times more likely to implement continuous documentation successfully.

For a complete overview of all diagramming tools, see our Online Diagram Tools Guide.

Getting Started: Core Syntax Reference

Accessing the Editor

1. Navigate to tools-online.app/tools/mermaid
2. The interface presents a code editor panel and a preview area

Core Mermaid Syntax

Diagram Type Declaration:

graph TD          # Flowchart (top-down)
sequenceDiagram  # Sequence diagram
classDiagram     # Class diagram
stateDiagram-v2  # State diagram
erDiagram        # Entity-relationship

Node Shapes:

A[Rectangle]
B(Rounded)
C{Diamond}
D((Circle))
E>Asymmetric]

Connections:

A --> B          # Arrow
A --- B          # Line
A -.-> B         # Dotted arrow
A ==> B          # Thick arrow
A -->|Label| B   # Labeled connection

Direction:

graph TD          # Top to bottom
graph LR          # Left to right
graph BT          # Bottom to top
graph RL          # Right to left

The Mermaid syntax reference provides comprehensive documentation for all diagram types.

Available Diagram Types

These formats align with ISO/IEC 19510:2013 standards for visualization and modeling.

Ready-to-Use Mermaid Templates

The Mermaid editor on tools-online.app includes pre-built templates for common workflows. Start with these professional templates — click any link to open the live editor with code and preview:

1. Flowchart Diagram

Best for: Process workflows, decision trees, algorithm documentation

2. Sequence Diagram

Best for: API interactions, system communication flows, message exchanges

3. Class Diagram

Best for: Object-oriented design, software architecture, database models

4. State Diagram

Best for: System states, workflow status, application lifecycle

5. C4 Diagram

Best for: Software architecture, system context, container diagrams. Endorsed by ThoughtWorks Technology Radar as a recommended architecture documentation approach.

6. Gantt Chart

Best for: Project timelines, sprint planning, resource scheduling. Recognized by the Project Management Institute's PMBOK® Guide as a standard scheduling tool.

7. Pie Chart

Best for: Market share, survey results, proportional data

8. Entity Relationship Diagram

Best for: Database schema, table relationships, data modeling

9. Git Graph

Best for: Version control history, branching strategies, release flows. According to GitLab's 2023 DevSecOps Report, teams with visualized branching strategies experience 35% fewer merge conflicts.

10. User Journey

Best for: User experience mapping, customer journeys, satisfaction tracking. Nielsen Norman Group's research indicates visual journey mapping increases team empathy by 74%.

Complete Diagram Type Guide

Flowcharts

Flowcharts are Mermaid's most popular diagram type, perfect for documenting workflows and decision logic.

Construction Steps:

1. Define direction: graph TD (top-down) or graph LR (left-right)
2. Create nodes: A[Text] for rectangles, B{Text} for diamonds
3. Connect nodes: A --> B for standard connections
4. Add labels: A -->|Label text| B

Example Code:

graph TD
    A[Start] --> B[Research Problem]
    B --> C{Solution Found?}
    C -->|Yes| D[Implement Solution]
    C -->|No| E[Modify Approach]
    E --> B
    D --> F[End]

Output:

graph TD
    A[Start] --> B[Research Problem]
    B --> C{Solution Found?}
    C -->|Yes| D[Implement Solution]
    C -->|No| E[Modify Approach]
    E --> B
    D --> F[End]

try here

Research from the Journal of Visual Languages & Computing shows flowcharts improve problem-solving efficiency by 43% compared to text-only instructions.

Advanced Flowchart Features

Subgraphs for Grouping:

graph TD
    A[Start]
    subgraph Processing
        B[Step 1]
        C[Step 2]
    end
    A --> B
    C --> D[End]

Styling Nodes:

graph LR
    A[Normal]
    B[Important]:::highlight
    classDef highlight fill:#f96,stroke:#333,stroke-width:4px

Complex Connections:

graph TD
    A --> B & C
    B & C --> D

Combine flowcharts with our Markdown editor for comprehensive documentation.

Sequence Diagrams

Sequence diagrams illustrate interactions between components over time.

Construction Steps:

1. Begin with sequenceDiagram declaration
2. Define participants: participant Name
3. Create message arrows: ParticipantA->>ParticipantB: Message text
4. Add notes: Note over ParticipantA: Note text

Example Code:

sequenceDiagram
    participant User
    participant System
    User->>System: Login Request
    System->>User: Request Credentials
    User->>System: Submit Credentials
    System->>System: Validate
    System->>User: Authentication Result

Output:

sequenceDiagram
    participant User
    participant System
    User->>System: Login Request
    System->>User: Request Credentials
    User->>System: Submit Credentials
    System->>System: Validate
    System->>User: Authentication Result

try here

This format follows the UML 2.5 specification for interaction diagrams.

Advanced Sequence Features

Loops and Alternatives:

sequenceDiagram
    loop Every Request
        Client->>Server: Send Data
    end
    alt Success
        Server->>Client: 200 OK
    else Failure
        Server->>Client: 500 Error
    end

Activation Boxes:

sequenceDiagram
    User->>+API: Request
    API->>+Database: Query
    Database-->>-API: Data
    API-->>-User: Response

Use sequence diagrams alongside D2 architecture diagrams for complete system documentation.

Class Diagrams

Class diagrams represent object relationships and structures.

Construction Steps:

1. Begin with classDiagram declaration
2. Define classes and attributes: class ClassName { +attribute: type +method() }
3. Establish relationships: ClassA <|-- ClassB for inheritance

Example Code:

classDiagram
    class User {
        +String username
        +String email
        +login()
        +logout()
    }
    class Admin {
        +manageUsers()
    }
    User <|-- Admin

Output:

classDiagram
    class User {
        +String username
        +String email
        +login()
        +logout()
    }
    class Admin {
        +manageUsers()
    }
    User <|-- Admin

try here

Entity Relationship Diagrams

ERDs visualize database structure and relationships.

Example Code:

erDiagram
    CUSTOMER ||--o{ ORDER : places
    CUSTOMER {
        string id
        string name
    }
    ORDER {
        string id
        date created_at
    }

Output:

erDiagram
    CUSTOMER ||--o{ ORDER : places
    CUSTOMER {
        string id
        string name
    }
    ORDER {
        string id
        date created_at
    }

try here

ERD notation follows database design standards, essential for documenting data models before implementation.

State Diagrams

State diagrams show system states and transitions.

Example Code:

stateDiagram-v2
    [*] --> Idle
    Idle --> Processing: Submit
    Processing --> Success: Complete
    Processing --> Error: Fail
    Success --> [*]
    Error --> Idle: Retry

Output:

stateDiagram-v2
    [*] --> Idle
    Idle --> Processing: Submit
    Processing --> Success: Complete
    Processing --> Error: Fail
    Success --> [*]
    Error --> Idle: Retry

try here

State diagrams conform to guidelines in NIST Special Publication 800-204 for system behavior representation.

User Journey Diagrams

User journey diagrams map experience flows and satisfaction.

Example Code:

journey
    title Onboarding Process
    section Registration
      Create account: 5: User
      Verify email: 3: User, System

Output:

journey
    title Onboarding Process
    section Registration
      Create account: 5: User
      Verify email: 3: User, System

try here

Gantt Charts

Gantt charts visualize project schedules and dependencies.

Example Code:

gantt
    title Project Timeline
    dateFormat YYYY-MM-DD
    section Planning
    Requirements : a1, 2023-05-01, 10d
    section Development
    Implementation : a2, after a1, 15d
    Testing : a3, after a2, 5d

Output:

gantt
    title Project Timeline
    dateFormat YYYY-MM-DD
    section Planning
    Requirements : a1, 2023-05-01, 10d
    section Development
    Implementation : a2, after a1, 15d
    Testing : a3, after a2, 5d

try here

Git Graphs

Git graphs visualize version control history.

Example Code:

gitGraph
    commit
    branch develop
    checkout develop
    commit
    checkout main
    merge develop

Output:

gitGraph
    commit
    branch develop
    checkout develop
    commit
    checkout main
    merge develop

try here

C4 Diagrams

C4 diagrams document software architecture at multiple abstraction levels.

Example Code:

C4Context
    title System Context diagram for Internet Banking System
    Enterprise_Boundary(b0, "BankBoundary") {
      Person(customer, "Banking Customer", "A customer of the bank")
      System(banking_system, "Internet Banking System", "Allows customers to view information about their bank accounts")

      Rel(customer, banking_system, "Uses")
    }
    System_Ext(email_system, "E-mail System", "The internal Microsoft Exchange e-mail system")
    System_Ext(mainframe, "Mainframe Banking System", "Stores all core banking information")

    Rel(banking_system, email_system, "Sends e-mail using")
    Rel(banking_system, mainframe, "Gets account information from")

Output:

C4Context
    title System Context diagram for Internet Banking System
    Enterprise_Boundary(b0, "BankBoundary") {
      Person(customer, "Banking Customer", "A customer of the bank")
      System(banking_system, "Internet Banking System", "Allows customers to view information about their bank accounts")

      Rel(customer, banking_system, "Uses")
    }
    System_Ext(email_system, "E-mail System", "The internal Microsoft Exchange e-mail system")
    System_Ext(mainframe, "Mainframe Banking System", "Stores all core banking information")

    Rel(banking_system, email_system, "Sends e-mail using")
    Rel(banking_system, mainframe, "Gets account information from")

try here

Component Diagrams

Component diagrams show system architectural elements and connections using flowchart syntax with subgraph grouping.

Example Code:

graph LR
    A[Web Client] --> B[API Gateway]
    B --> C[Authentication Service]
    subgraph Backend Services
        C
    end

Output:

graph LR
    A[Web Client] --> B[API Gateway]
    B --> C[Authentication Service]
    subgraph Backend Services
        C
    end

Pie Charts

Pie charts show proportional data distribution.

Example Code:

pie title Market Share
    "Product A" : 42.5
    "Product B" : 28.9
    "Product C" : 28.6

Output:

pie title Market Share
    "Product A" : 42.5
    "Product B" : 28.9
    "Product C" : 28.6

try here

Resource Breakdown Structure (RBS)

Resource breakdown structures illustrate project resources and their relationships, providing a structured overview for successful project completion.

Example Code:

graph TD
    A[Project Resources] --> B[Human Resources]
    A --> C[Material Resources]
    B --> B1[Project Manager]
    B --> B2[Technical Team]

Output:

graph TD
    A[Project Resources] --> B[Human Resources]
    A --> C[Material Resources]
    B --> B1[Project Manager]
    B --> B2[Technical Team]

try here

The RBS is recognized in the PMBOK® Guide as a critical resource planning tool.

AI-Powered Mermaid Diagram Generation

Creating diagrams from scratch can be time-consuming. Our AI Diagram Assistant generates complete Mermaid syntax from natural language.

How to Use AI for Mermaid Diagrams

1. Get your API key from AIML API
2. Click "Settings" icon (located lower left) in any tools-online.app tool.
   
3. Add API key and save.

Step 2: Open AI Chat

1. Click the AI Chat button (located lower left)
2. Choose "Generate" mode and provide a natural language description for the diagram.
   

Step 3: Describe Your Diagram

Example Prompts:

Flowcharts:

• "Create a user login flow with email verification"
• "Generate a decision tree for customer support triage"
• "Make a deployment process flowchart with rollback"

Sequence Diagrams:

• "Show API authentication flow with JWT tokens"
• "Create microservices communication diagram"
• "Generate payment processing sequence with third-party gateway"

Class Diagrams:

• "Design an e-commerce system with products, orders, and users"
• "Create a blog platform class structure"
• "Generate a library management system ERD"

Project Planning:

• "Make a Gantt chart for 3-month software project"
• "Create a Git branching strategy diagram"
• "Generate a user journey for mobile app onboarding"

The AI creates production-ready Mermaid code that you can immediately render, customize, and export.

For comprehensive AI integration, see our AI Integration Guide and how AI enhances diagramming.

Mermaid vs Other Diagram Tools

Different tools excel at different tasks. Here's when to choose Mermaid:

Use Mermaid when you need:

• Documentation diagrams for GitHub/GitLab
• Quick flowcharts and sequence diagrams
• Version-controllable diagram source
• Minimal syntax and fast creation
• Native markdown integration
• Multiple diagram types from one tool

Use D2 when you need:

• Precise architecture diagrams
• Custom positioning and layout
• Professional infrastructure visualization
• Cloud system design (AWS, Azure, GCP)

Use GraphViz when you need:

• Complex dependency graphs
• Network topology with 100+ nodes
• Automatic graph layout algorithms
• Mathematical graph theory applications

Use Gnuplot when you need:

• Scientific data visualization
• Mathematical function plotting
• Publication-quality charts
• 2D and 3D data plots

Feature Comparison

For complete tool comparisons, explore our comprehensive diagram tools guide and best free diagram editors.

GitHub Integration, Export, and Best Practices

Using Mermaid in GitHub

Mermaid's killer feature is native GitHub support. According to GitHub's documentation, Mermaid diagrams render automatically in markdown files.

Simply wrap your Mermaid code in markdown code blocks:

```mermaid
graph TD
    A[Start] --> B[End]
```

GitHub renders this as a visual diagram in:

• README.md files
• Issue descriptions
• Pull request comments
• Wiki pages
• Gists

Export Options

Our online editor provides multiple export formats:

SVG (Recommended)

• Scalable vector graphics
• Crisp at any resolution
• Small file sizes
• Best for documentation

PNG

• Raster images
• Universal compatibility
• Good for presentations
• Quick sharing

Source Code

• Copy Mermaid syntax
• Version control in Git
• Embed in documentation
• Share with teams

All processing happens client-side in your browser — your diagrams never leave your device. Learn more about our privacy-first approach.

Diagram Creation Guidelines

1. Keep diagrams focused — One diagram should communicate one concept. Split complex systems into multiple focused diagrams.
2. Use descriptive labels — Avoid generic labels like A --> B. Use User[Customer] --> Login[Authentication System].
3. Maintain consistent direction — Choose one flow direction per diagram (top-down or left-right) and stick with it.
4. Add comments for complex logic — Use %% comments to explain non-obvious sections.
5. Version control your diagrams — Store Mermaid source code in your repository alongside documentation.
6. Test in target environment — If embedding in GitHub, test rendering there. Different platforms may have slightly different Mermaid version support.

Edward Tufte, visualization expert and author of "The Visual Display of Quantitative Information", emphasizes that "clarity in thinking is very much like clarity in the display of data."

Troubleshooting Common Issues

The Mermaid GitHub repository provides solutions to common rendering issues.

Common Mermaid Questions

How do I embed Mermaid in my website? Include the Mermaid JavaScript library and wrap your diagram code in <div class="mermaid"> tags. See the official Mermaid integration guide for details.

Can I customize colors and styles? Yes, using CSS classes and theme directives:

%%{init: {'theme':'dark'}}%%
graph TD
    A:::customStyle --> B
    classDef customStyle fill:#f96,stroke:#333

Do Mermaid diagrams work in Notion? Notion doesn't support native Mermaid rendering. Export as SVG or PNG and embed as images instead.

Can I create custom diagram types? Mermaid supports 10+ built-in types. For custom visualizations beyond these, consider D2 or GraphViz.

How do I fix "Syntax Error" messages? Common causes:

• Missing quotes around labels with spaces
• Incorrect arrow syntax (-> vs -->)
• Unmatched brackets or braces
• Reserved keywords used as node IDs

Use our live editor's syntax highlighting to catch errors as you type.

Can I animate Mermaid diagrams? Static rendering is standard. For animated visualizations, export frames as separate diagrams or use JavaScript animation libraries post-rendering.

Is Mermaid suitable for large enterprise systems? For systems with 50+ components, consider breaking into multiple diagrams or using GraphViz for better layout control of complex graphs.

Conclusion and Next Steps

Mermaid offers an efficient, text-based approach to diagram creation that integrates seamlessly with documentation processes. Its accessible syntax reduces the learning curve while providing powerful visualization capabilities.

Core Benefits:

• Version control compatibility: Diagrams can be tracked alongside code
• Consistency in documentation: Standardized visual language
• Reduced maintenance overhead: Update diagrams with simple text edits
• Accessibility: Create diagrams without specialized graphic design tools

Start Creating:

• Open Mermaid Editor - Try ready-made templates
• AI Diagram Generation - Generate diagrams from descriptions

Explore More Tools:

• Complete Diagram Tools Guide - All visualization options
• D2 Editor - Architecture diagrams
• GraphViz Editor - Network graphs
• Gnuplot Editor - Data visualization

Documentation Tools:

• Markdown Editor - Write documentation
• Code Compare - Track diagram changes
• JSON Tool - Process structured data
• YAML Tool - Configuration files

Learn More:

• How AI Enhances Diagramming - Automate diagram creation
• Browser-Based Tools Benefits - Why online tools matter
• Best Free Diagram Editors - Tool comparisons

Additional Resources:

• Mermaid Documentation - Comprehensive syntax reference
• Mermaid GitHub Repository - Latest updates and community support

Access the Mermaid diagram tool at tools-online.app/tools/mermaid