UML Diagram Editor Online — Free PlantUML & Mermaid Editor with AI

How to Create UML Diagrams Online for Free

Create a supported diagram with PlantUML or Mermaid syntax. You can also request an AI draft, then review its syntax and meaning. FindUtils UML Diagram Editor supports both PlantUML and Mermaid.js with AI-powered generation, live preview, 40+ themes, and SVG/PNG export. No signup, no downloads, no Java installation required.

Whether you need a sequence diagram for an API flow, a class diagram for your data model, or a Gantt chart for project planning, this guide walks you through creating professional diagrams in minutes -- by writing code, asking AI, or both.

Why Use Text-Based Diagrams Instead of Drag-and-Drop?

Text-based diagrams solve problems that visual editors create. Here's why development teams are switching:

• Version control friendly -- Diagram code lives in Git alongside your application code. Every change is trackable in pull requests
• Text edits -- Change a relationship by editing a line such as A -> B : request
• AI drafts -- Describe the required relationships, then check the generated diagram
• Consistent formatting -- The renderer handles layout automatically. No more hand-aligning shapes
• Easy collaboration -- Share diagram code in Slack, paste it in documentation, include it in code reviews
• Repeatable source -- Store the diagram text and renderer version. A renderer or theme change can alter the layout

Text-based diagrams can be stored beside documentation and reviewed as text changes. Choose the notation that your readers and rendering tools support.

Where diagram data goes

Mermaid rendering runs in the browser. PlantUML rendering sends encoded diagram text to the configured rendering service. Encoding that text is not encryption. AI features send prompt context to the selected provider.

The editor can retain tabs and conversation history in browser storage. Remove confidential names and credentials before using remote rendering, AI, or shareable links.

Step 1: Open the UML Diagram Editor

Go to the FindUtils UML Diagram Editor. You'll see a split-screen layout: code editor on the left, live diagram preview on the right. An AI chat panel is available at the bottom for conversational diagram building.

The editor starts in Mermaid mode. Select a template when you need a sample diagram. You can type, and the preview updates after a brief pause -- no need to click a "render" button.

Step 2: Choose Your Diagram Language

Click the PlantUML or Mermaid toggle button in the toolbar. Each language has different strengths:

PlantUML is ideal when you need:

• 15+ diagram types (sequence, class, use case, activity, component, state, object, deployment, timing, ER, and more)
• 40+ built-in themes for customization
• Complex layouts with detailed styling options

Mermaid is ideal when you need:

• Local rendering after the page and required renderer load
• GitHub/GitLab native support (renders in README files automatically)
• Flowcharts, Gantt charts, and pie charts alongside UML types

Step 3: Generate Diagrams with AI

The editor includes a built-in AI assistant that can generate complete diagrams from natural language descriptions. You have three ways to use AI:

Quick AI Generation

Click the AI Generate button in the toolbar. Type a description like "Create a sequence diagram showing a user login flow with OAuth2" and the AI generates the complete PlantUML or Mermaid code instantly. The generated code appears in your editor with live preview -- you can edit and refine it further.

AI Chat for Iterative Refinement

Open the AI Chat panel at the bottom of the editor. This is a full conversational interface where you can:

• Ask the AI to create a diagram from scratch: "Make a class diagram for an e-commerce system with User, Product, Order, and Payment classes"
• Refine existing diagrams: "Add error handling to this sequence diagram" or "Make the arrows dashed between the API and database"
• Fix errors: Click "Fix with AI" when a diagram has rendering errors -- the AI sees your code and the error, then suggests a fix
• Build incrementally: Each message preserves context, so you can say "Now add a Cart class" and the AI updates the existing diagram

The AI chat maintains up to 20 messages of conversation history and automatically includes your current diagram code as context.

AI Provider Options

The settings show the available AI providers. Check the selected model, request limits, and provider charges before you send a prompt. A personal API key does not remove provider limits.

The next section uses text examples that you can review without an AI request.

Step 4: Write Your Diagram Code

Use the built-in Templates dropdown to start with a working example, then modify it. The editor supports Tab key for indentation and line numbers for easy reference.

Sequence Diagram Example (PlantUML)

@startuml
actor User
participant "API Gateway" as GW
participant "Auth Service" as Auth
database "User DB" as DB

User -> GW : POST /login
GW -> Auth : Validate credentials
Auth -> DB : SELECT user
DB --> Auth : user record
Auth --> GW : JWT token
GW --> User : 200 OK { token }
@enduml

Flowchart Example (Mermaid)

graph TD
 A[Start] --> B{Valid Input?}
 B -->|Yes| C[Process]
 B -->|No| D[Show Error]
 C --> E[Save to DB]
 D --> A
 E --> F[Send Response]

Step 5: Use Version History

When working with AI Chat, the editor automatically saves up to 10 previous versions of your diagram. Each time you apply an AI suggestion, the previous version is saved. Click Version History to browse and restore any earlier version with one click -- no undo/redo needed.

You can also export your entire AI conversation as Markdown for documentation or sharing with your team.

Step 6: Customize with Themes

In PlantUML mode, use the Theme dropdown to apply visual styles. Popular themes include:

• Blueprint -- Technical blueprint aesthetic with blue grid background
• Sketchy -- Hand-drawn look, great for presentations
• Cerulean -- Clean, professional blue theme
• Hacker -- Dark theme with green accents
• Vibrant -- Colorful, high-contrast for slides

Mermaid uses its own theme system. The editor selects the dark or default theme from the current page appearance.

Step 7: Export Your Diagram

The toolbar offers multiple export options:

• Export SVG -- Vector format that scales to any size. Ideal for documentation and web use
• Download PNG -- Raster image at 2x resolution for crisp display on high-DPI screens
• Copy as Markdown -- Generates ![Diagram](url) for PlantUML or a fenced code block for Mermaid. Paste directly into GitHub READMEs
• Share -- Sends the diagram to the sharing service and copies a URL with a share identifier. Optional chat history can also be included

Real-World Scenarios

Scenario 1: AI-Assisted API Documentation

A backend developer opens the AI chat and types "Create a sequence diagram showing the OAuth2 authorization code flow with a client app, auth server, and resource server." The AI generates a complete PlantUML diagram in seconds. The developer says "Add token refresh flow" and the AI updates the diagram while preserving the original flow. This is an illustrative workflow, not a measured timing result.

Scenario 2: Architecture Review

A tech lead creates a component diagram showing the system architecture before a design review. They share the URL with the team on Slack. Colleagues can open it, fork it to a new tab, and suggest modifications.

Scenario 3: Sprint Planning with AI

A project manager tells the AI "Create a Mermaid Gantt chart for a 2-week sprint with frontend, backend, testing, and deployment phases." The AI generates the chart instantly. The PM refines dates in the code and copies the Markdown into the GitHub sprint issue.

Scenario 4: Fixing Broken Diagrams

A junior developer pastes a PlantUML diagram that has syntax errors. Instead of debugging the syntax manually, they click "Fix with AI" -- the AI analyzes the error, identifies the issue, and provides corrected code. The developer checks the suggested change, then renders the diagram again.

Scenario 5: Database Design

A database architect uses the ER diagram template to model the schema. They export it as PNG and include it in the pull request that creates the migration files.

UML Diagram Editor Features

Common Mistakes When Creating UML Diagrams

Mistake 1: Forgetting the @startuml / @enduml Tags

PlantUML requires @startuml at the beginning and @enduml at the end. Missing either one produces no output. Mermaid does not use these tags. Review AI-generated syntax before applying it.

Mistake 2: Using Wrong Arrow Syntax

PlantUML uses -> for solid arrows and --> for dashed arrows. Mermaid uses --> for solid and -.-> for dashed. Mixing syntaxes between languages causes rendering errors. If you get confused, use the "Fix with AI" button.

Mistake 3: Not Using Aliases for Long Names

Instead of repeating "Authentication Service" throughout your diagram, define an alias: participant "Authentication Service" as Auth. This makes diagrams shorter and more readable.

Mistake 4: Overloading a Single Diagram

If your diagram has more than 15 participants or 30 interactions, split it into multiple focused diagrams. Use the multi-tab feature to keep related diagrams together.

Mistake 5: Writing Diagrams from Scratch When AI Can Help

Start with a template or an AI draft when that reduces repeated work. Check each participant, relationship, and error path before you use the result.

Tools Used in This Guide

• UML Diagram Editor -- Create sequence, class, activity, state, and ER diagrams with PlantUML or Mermaid syntax, with AI-powered generation and chat
• ER Diagram Designer -- Visual drag-and-drop entity-relationship diagram builder with SQL export
• State Machine Designer -- Interactive state machine editor with transition visualization
• Architecture Diagram -- System architecture diagram builder with tech stack component palette
• Markdown Previewer -- Preview Markdown with Mermaid diagram support built in

FAQ

Q1: Is the UML Diagram Editor free to use? A: The manual editor is available without signup. Mermaid rendering runs in the browser. PlantUML rendering uses a remote service. Check the AI settings for current limits.

Q2: How does the AI diagram generation work? A: Describe what you want in plain English (e.g., "Create a class diagram for a blog system with User, Post, Comment, and Tag classes") and the AI generates complete PlantUML or Mermaid code. The AI chat also supports iterative refinement -- you can say "add a Category class" and the AI updates the existing diagram.

Q3: When should I use this UML diagram editor? A: It supports both PlantUML and Mermaid syntax in a single editor with AI generation, conversational AI chat, 40+ themes, multi-tab persistence, version history, shareable URLs, and dual SVG/PNG export.

Q4: Do I need to install Java to use PlantUML? A: No. The editor uses the public PlantUML server for rendering, so you don't need Java, Graphviz, or any local installation. Just open the editor in your browser and start typing.

Q5: Can I use Mermaid diagrams in GitHub? A: Yes. GitHub natively renders Mermaid code blocks in Markdown files. Use the "Copy MD" button to copy your Mermaid diagram as a fenced code block, then paste it directly into any GitHub README or issue.

Q6: Are my diagrams saved? A: Yes. All diagrams are automatically saved to your browser's local storage using the multi-tab system. Your work persists across page reloads. AI conversations are also saved locally. You can share diagrams via URL or export them as files.

Q7: What AI models power the diagram generation? A: The settings show the available AI providers. Provider availability, quotas, and model selection can change. A personal API key does not remove provider limits or charges.

Q8: Is it safe to use online diagram editors for sensitive designs? A: Mermaid rendering runs locally. PlantUML, AI, and sharing send diagram or prompt data to remote services. Browser storage can retain tabs and chat history. Use approved handling rules for confidential designs.

Next Steps

• ER Diagram Designer -- If you need a visual drag-and-drop editor for database schemas with SQL export
• State Machine Designer -- For interactive state machine diagrams with transition simulation
• Architecture Diagram -- Build system architecture diagrams with pre-built cloud and infrastructure components
• Markdown Previewer -- Preview your documentation with embedded Mermaid diagrams before committing