PlantUML vs Mermaid: Which Diagram Tool Should You Use?

Last Updated: January 15, 2025

When it comes to creating technical diagrams, especially Entity-Relationship (ER) diagrams for database design, developers have several options. Two of the most popular text-based diagram tools are PlantUML and Mermaid. Both allow you to create diagrams using simple text syntax instead of drag-and-drop interfaces, but they have distinct differences that make each suitable for different use cases.

In this comprehensive guide, we'll compare PlantUML and Mermaid across multiple dimensions, provide syntax examples for ER diagrams, and help you decide which tool is right for your project.

Introduction: Text-Based Diagram Tools

Traditional diagram tools like Microsoft Visio, Lucidchart, or Draw.io require you to manually draw shapes, connect lines, and position elements. While these tools offer visual flexibility, they come with several drawbacks:

Text-based diagram tools solve these problems by allowing you to define diagrams in plain text files. This approach offers several advantages:

PlantUML and Mermaid are the two leading tools in this space. Let's explore each one in detail.

What is PlantUML?

PlantUML is an open-source tool that allows you to create UML diagrams from plain text descriptions. Created in 2009, it has become one of the most mature and feature-rich diagram tools available.

Key Features

Advantages

Disadvantages

PlantUML ER Diagram Syntax Example

@startuml
' Define entities
entity "Users" as users {
    * id : INT <>
    --
    * username : VARCHAR(50)
    * email : VARCHAR(100)
    password_hash : VARCHAR(255)
    created_at : TIMESTAMP
    updated_at : TIMESTAMP
}

entity "Orders" as orders {
    * id : INT <>
    --
    * user_id : INT <>
    order_number : VARCHAR(50)
    total_amount : DECIMAL(10,2)
    status : VARCHAR(20)
    created_at : TIMESTAMP
}

entity "OrderItems" as order_items {
    * id : INT <>
    --
    * order_id : INT <>
    * product_id : INT <>
    quantity : INT
    unit_price : DECIMAL(10,2)
}

entity "Products" as products {
    * id : INT <>
    --
    * name : VARCHAR(100)
    description : TEXT
    price : DECIMAL(10,2)
    stock_quantity : INT
}

' Define relationships
users ||--o{ orders : places
orders ||--|{ order_items : contains
products ||--o{ order_items : "included in"
@enduml

What is Mermaid?

Mermaid is a JavaScript-based diagramming and charting tool that uses Markdown-inspired text definitions to create diagrams dynamically in the browser. Launched in 2014, it has gained massive popularity due to its simplicity and native integration with many platforms.

Key Features

Advantages

Disadvantages

Mermaid ER Diagram Syntax Example

erDiagram
    USERS ||--o{ ORDERS : places
    ORDERS ||--|{ ORDER_ITEMS : contains
    PRODUCTS ||--o{ ORDER_ITEMS : "included in"
    
    USERS {
        int id PK
        string username UK
        string email UK
        string password_hash
        timestamp created_at
        timestamp updated_at
    }
    
    ORDERS {
        int id PK
        int user_id FK
        string order_number
        decimal total_amount
        string status
        timestamp created_at
    }
    
    ORDER_ITEMS {
        int id PK
        int order_id FK
        int product_id FK
        int quantity
        decimal unit_price
    }
    
    PRODUCTS {
        int id PK
        string name
        text description
        decimal price
        int stock_quantity
    }

Feature Comparison Table

Feature PlantUML Mermaid
Rendering Method Server-side (Java + Graphviz) or remote server Client-side (JavaScript in browser)
Syntax Complexity Moderate to complex Simple and intuitive
Diagram Types 20+ types (comprehensive UML support) 12+ types (covers most common needs)
Theme Support Extensive theming and customization Limited theme options
Offline Use Requires Java and Graphviz installation Works offline once loaded
GitHub Compatibility Requires extensions or actions Native support in Markdown
Learning Curve Steeper due to extensive features Gentle, easy to start
Setup Requirements Java JDK, Graphviz (for local rendering) None (just include JavaScript library)
Performance Slower for complex diagrams Fast for most use cases
Community Size Large, mature community Rapidly growing community
Documentation Comprehensive and detailed Good and improving
IDE Support Plugins for most major IDEs Growing IDE plugin ecosystem
Best For Complex UML diagrams, enterprise projects Documentation, README files, quick diagrams

ER Diagram Syntax Comparison

To better understand the differences, let's look at how the same database structure would be represented in both PlantUML and Mermaid.

Database Structure

Consider a simple e-commerce database with four tables: Users, Orders, OrderItems, and Products.

PlantUML Approach

PlantUML uses the entity keyword to define tables and supports detailed column definitions with data types and constraints. Relationships are defined using Crow's Foot notation with clear syntax for cardinality.

Note: PlantUML's ER diagram syntax is more verbose but provides greater control over layout and appearance. It's particularly useful when you need precise control over diagram structure.

Mermaid Approach

Mermaid uses a cleaner, more concise syntax. The erDiagram keyword starts the diagram, and relationships are defined in a natural language-like format. Column definitions are simpler but still effective.

Note: Mermaid's syntax is easier to read and write, making it ideal for quick documentation and team collaboration where everyone needs to understand the diagram syntax.

When to Use PlantUML vs Mermaid

Choose PlantUML When:

Choose Mermaid When:

Using Both in Free ER Diagram Tool

At Free ER Diagram, we believe you shouldn't have to choose between PlantUML and Mermaid. That's why our online tool supports both formats, giving you the flexibility to use whichever syntax you prefer.

Our tool offers:

Whether you're a PlantUML veteran who loves its power or a Mermaid enthusiast who appreciates its simplicity, Free ER Diagram accommodates your workflow. You can even switch between syntaxes to see which works better for your specific use case.

Try it now: Visit Free ER Diagram and start creating professional ER diagrams in minutes, using either PlantUML or Mermaid syntax.

Conclusion

Both PlantUML and Mermaid are excellent tools for creating text-based diagrams, each with its own strengths. PlantUML offers more power and flexibility, making it ideal for complex, enterprise-level diagrams. Mermaid provides simplicity and ease of use, perfect for documentation and quick diagram creation.

The best choice depends on your specific needs:

Ultimately, the goal is to create clear, maintainable diagrams that effectively communicate your database structure. Both tools can help you achieve this—choose the one that best fits your workflow and team requirements.

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