Mermaid Diagram Tool Online - Free Flowchart & UML Editor

Documentation without diagrams is incomplete. Whether you're explaining workflows, designing systems, or mapping user journeys, Mermaid turns simple text into professional diagrams that integrate seamlessly with your documentation.

At tools-online.app, we've built a free, browser-based Mermaid editor that makes diagram creation as simple as writing markdown. No drag-and-drop complexity, no binary file formats—just clean, version-controllable text that renders beautiful visualizations.

This guide shows you how to create flowcharts, sequence diagrams, and 10+ other diagram types using Mermaid's intuitive syntax. You'll learn when to choose Mermaid over alternatives like D2 or GraphViz, and how AI can generate diagrams from simple descriptions.

Table of Contents

1. What is Mermaid and Why Use It Online?
2. Getting Started: Mermaid Basics
3. Ready-to-Use Mermaid Templates
4. Creating Process Diagrams
5. Creating Interaction Diagrams
6. Creating Structural Diagrams
7. AI-Powered Mermaid Diagram Generation
8. Mermaid vs Other Diagram Tools
9. GitHub Integration and Export
10. Best Practices for Mermaid Diagrams
11. Common Mermaid Questions
12. Next Steps: Master Diagram-as-Code

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, 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: Mermaid Basics

Mermaid uses markdown-inspired syntax that's immediately readable. Here's your first diagram:

graph TD
    A[Start] --> B[Process]
    B --> C[End]

This creates a top-down flowchart with three connected boxes. The code appears on the left side of our editor, and the rendered diagram displays on the right.

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.

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

Visualizes step-by-step processes with decision points and multiple paths. Perfect for documenting business logic, user flows, or system processes.

2. Sequence Diagram

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

Shows how components interact over time with messages flowing between participants. Essential for documenting distributed systems and microservices.

3. Class Diagram

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

Represents classes, attributes, methods, and their relationships. Crucial for documenting code structure and inheritance hierarchies.

4. State Diagram

Best for: System states, workflow status, application lifecycle

Shows how systems transition between different states based on events or conditions. Used in embedded systems, game development, and workflow engines.

5. C4 Diagram

Best for: Software architecture, system context, container diagrams

Documents architecture at multiple abstraction levels following the C4 model. Endorsed by ThoughtWorks Technology Radar as a recommended architecture documentation approach.

6. Gantt Chart

Best for: Project timelines, sprint planning, resource scheduling

Visualizes project schedules with tasks, durations, and dependencies. 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

Shows data distribution as percentage slices. Simple yet effective for presenting categorical proportions.

8. Entity Relationship Diagram

Best for: Database schema, table relationships, data modeling

Visualizes database structure with entities and their connections. Follows standards described in academic database design literature.

9. Git Graph

Best for: Version control history, branching strategies, release flows

Visualizes Git commit history and branch structures. 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

Maps user interactions with satisfaction scores at each step. Nielsen Norman Group's research indicates visual journey mapping increases team empathy by 74%.

Creating Process Diagrams

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

Basic Flowchart Structure

graph TD
    A[Start] --> B{Decision?}
    B -->|Yes| C[Action 1]
    B -->|No| D[Action 2]
    C --> E[End]
    D --> E

Try this flowchart live →

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.

Creating Interaction Diagrams

Sequence diagrams visualize how system components communicate over time.

Sequence Diagram Basics

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

Try this sequence diagram →

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.

Creating Structural Diagrams

Class diagrams document object-oriented designs and database structures.

Class Diagram Syntax

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

Try this class diagram →

Entity-Relationship Diagrams

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

Try this ERD →

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

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 2025 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:

Feature Comparison

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

GitHub Integration and Export

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

Using Mermaid in GitHub

Simply wrap your Mermaid code in markdown code blocks:

markdown

```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.

Best Practices for Mermaid Diagrams

Create maintainable, professional diagrams by following these guidelines:

1. Keep Diagrams Focused One diagram should communicate one concept. Split complex systems into multiple focused diagrams rather than creating overwhelming visualizations.

// Good - focused on authentication
graph TD
    A[User] --> B[Login]
    B --> C{Valid?}
    C -->|Yes| D[Dashboard]
    C -->|No| B

2. Use Descriptive Labels

// Avoid generic labels
A --> B

// Use meaningful descriptions
User[Customer] --> Login[Authentication System]

3. Maintain Consistent Direction Choose one flow direction per diagram (top-down or left-right) and stick with it for visual consistency.

4. Add Comments for Complex Logic

graph TD
    %% Main authentication flow
    A[User] --> B{Authenticated?}
    %% Handle authentication failure
    B -->|No| C[Login Page]

5. Version Control Your Diagrams Store Mermaid source code in your repository alongside documentation. Use Code Compare to track diagram changes over time.

6. Test in Target Environment If embedding in GitHub, test rendering there. Different platforms may have slightly different Mermaid version support.

7. Combine with Written Documentation Diagrams complement but don't replace text. Use our Markdown editor to create comprehensive documentation with embedded diagrams.

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."

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.

Next Steps: Master Diagram-as-Code

Ready to create professional documentation diagrams?

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

Mermaid transforms technical documentation by making diagrams as easy to create and maintain as code. Whether you're documenting APIs, designing workflows, or planning projects, our free online editor delivers GitHub-native diagrams without installation complexity.

Questions or feedback? All tools on tools-online.app are completely free with no account required. Start creating diagrams today.