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ER Diagram Builder

Design professional Entity Relationship (ER) diagrams directly in your browser with drag-and-drop editing, PK/FK support, Crow's Foot cardinality, smart relationship routing, automatic local saving, and PNG export. Perfect for database design, SQL modeling, and software architecture.

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Tool Details

Build Professional Entity Relationship Diagrams Directly in Your Browser

Designing a well-structured database starts with a clear Entity Relationship (ER) diagram. Whether you're building a web application, designing a SaaS product, planning a database schema, or learning database concepts, ER diagrams help visualize how tables relate to each other before writing a single SQL query.

The ER Diagram Builder on CoolDev.Tools is a browser-based editor that lets you create professional database diagrams quickly and intuitively.

You can:

  • create entities (tables)
  • add unlimited attributes
  • mark Primary Keys (PK) and Foreign Keys (FK)
  • connect related tables
  • define Crow's Foot cardinality
  • label relationships
  • export diagrams as PNG

Everything is saved automatically in your browser. No account, login, or server uploads are required.

Whether you're designing a small project or planning a complex relational database, this tool provides everything you need for clean and accurate ER diagrams.

What is an ER Diagram?

An Entity Relationship (ER) Diagram is a visual representation of a database structure.

Instead of reading SQL statements, developers can understand:

  • tables
  • columns
  • primary keys
  • foreign keys
  • relationships
  • cardinality

at a glance.

ER diagrams are commonly used before database development begins because they make planning much easier.

Why Use an ER Diagram?

Planning a database visually helps prevent design mistakes before implementation.

Benefits include:

  • better database planning
  • easier collaboration
  • fewer schema mistakes
  • improved documentation
  • simpler maintenance

Whether you're building an e-commerce platform, CRM, blog, LMS, or SaaS application, an ER diagram serves as the blueprint for your database.

Create Database Tables Easily

Each entity represents a database table.

You can:

  • add new entities
  • rename tables
  • move entities freely
  • organize layouts visually

Simply click anywhere on the canvas to place new tables from the sidebar.

Add Unlimited Attributes

Every table can contain unlimited attribute rows.

Examples include:

  • id
  • email
  • created_at
  • price
  • status

Rows can be added or removed at any time while designing your schema.

Primary Key (PK) Support

Each attribute can be marked as a Primary Key (PK).

Primary keys uniquely identify every row inside a table.

Example:

id (PK)

The builder visually distinguishes PK fields, making table structures easier to understand.

Foreign Key (FK) Support

Attributes can also be marked as Foreign Keys (FK).

Foreign keys establish relationships between tables.

Example:

user_id (FK)

Only PK and FK rows expose connection points, keeping diagrams clean while encouraging proper relational modeling.

Smart Relationship Connections

Creating relationships between entities is simple.

Just drag from a PK or FK row and connect it to another compatible field.

The editor automatically provides:

  • generous snap areas
  • forgiving hit detection
  • smooth connection creation

No pixel-perfect clicking is required.

Dynamic Connection Routing

Unlike many online ER tools, connections automatically adjust while you move tables.

The editor intelligently selects the best connection side so that lines avoid cutting through entities.

Connections use:

  • right-angle routing
  • clean bends
  • readable layouts

without requiring manual adjustments.

Reconnect Existing Relationships

Relationships can be modified without deleting them.

Simply drag either endpoint of an existing connection and attach it to another PK or FK field.

This makes schema refactoring much faster.

Crow's Foot Cardinality Support

The builder supports proper Crow's Foot notation, which is the industry standard for representing database relationships.

Supported cardinalities include:

  • Exactly One
  • Zero or One
  • One or Many
  • Zero or Many

Each end of a relationship can be configured independently, allowing accurate modeling of real database relationships.

Cardinality indicators can be changed with a single click.

Relationship Labels

Relationships often benefit from descriptive labels.

Simply double-click any connection line to add or edit text.

Examples:

  • owns
  • contains
  • belongs to
  • manages
  • references

Labels improve readability for larger database diagrams.

Undo, Redo and Productivity Features

Database design is an iterative process.

The editor includes productivity features such as:

  • Undo
  • Redo
  • Copy
  • Paste
  • Multi-select

These features make editing large diagrams significantly faster.

Smart Alignment Guides

While moving entities, alignment guides automatically appear.

These guides help:

  • align tables neatly
  • maintain consistent spacing
  • create professional-looking diagrams

without manually adjusting positions.

Automatic Local Saving

One of the biggest advantages of the ER Diagram Builder is automatic saving.

Every change is stored in your browser's local storage.

That means:

  • no account required
  • no login
  • no manual save button
  • no server uploads

Even if you refresh the page, your diagram remains exactly where you left it.

Privacy First

Unlike many cloud-based diagram tools, this editor keeps your data on your own device.

Your diagrams are never uploaded to external servers.

This makes it suitable for:

  • confidential database schemas
  • internal company projects
  • prototype designs
  • offline-friendly workflows

Export Your Diagram

Once your database design is complete, you can export it as a high-quality PNG image.

PNG exports are ideal for:

  • documentation
  • presentations
  • architecture diagrams
  • project reports
  • client discussions

How to Use the ER Diagram Builder

Using the editor is straightforward.

Step 1: Create Entities

Add tables from the sidebar.

Step 2: Add Attributes

Insert columns for each entity.

Step 3: Mark PK and FK Fields

Identify primary and foreign keys.

Step 4: Connect Related Tables

Drag from PK/FK rows to create relationships.

Step 5: Configure Cardinality

Click relationship endpoints to cycle through Crow's Foot notation.

Step 6: Label Relationships

Double-click a connection to add descriptive text.

Step 7: Export as PNG

Download your finished ER diagram.

Common Use Cases

The ER Diagram Builder is useful for many types of projects.

SaaS Applications

Plan scalable relational databases.

E-Commerce Platforms

Model products, orders, customers, and payments.

Learning SQL

Students can understand relational databases visually.

Software Documentation

Create diagrams for technical documentation.

Database Migration

Visualize existing schemas before making structural changes.

Frequently Asked Questions (FAQs)

What is an ER diagram?

An ER diagram is a visual representation of database tables, attributes, and relationships.

Can I create unlimited tables?

Yes. You can add as many entities and attributes as your project requires.

Does the tool support Crow's Foot notation?

Yes. It supports Exactly One, Zero or One, One or Many, and Zero or Many cardinalities independently on both ends of every relationship.

Is my diagram uploaded anywhere?

No. Everything is stored locally in your browser using local storage.

Can I export my diagram?

Yes. Completed diagrams can be exported as PNG images.

Conclusion

A well-designed database begins with a clear ER diagram. Visualizing entities, relationships, primary keys, foreign keys, and cardinality before implementation reduces development time and prevents costly design mistakes.

The ER Diagram Builder on CoolDev.Tools provides a modern browser-based environment for designing relational databases with smart connections, Crow's Foot notation, automatic local saving, and PNG export. Whether you're building a personal project, an enterprise application, or learning database design, this tool makes creating professional ER diagrams simple, fast, and completely private.