# UML class diagram maker — classes & relations, no upload

> Free UML class diagram maker: describe classes, fields, methods, and relationships in Mermaid and render a diagram — live preview, local export, no sign-up.

Describe classes with their fields, methods, and relationships in Mermaid and get a UML class diagram with live preview and local export. No sign-up, and nothing is uploaded.

A class diagram documents structure: the types in a system, what they hold and do, and how they relate. Sketching one in Mermaid is faster than a drawing tool because you write the classes and connectors and let the layout resolve.

The payoff is maintenance — a text diagram committed alongside the code stays accurate, where an exported image quietly rots. Visibility markers and multiplicity make it a real design artifact, not just a picture.

For database tables use the ER diagram, for behavior over time use the Sequence diagram, and to mix engines use the Playground.

## How to

1. **Define classes.** List each class with its fields and methods.
2. **Add relationships.** Connect classes with inheritance, association, or composition.
3. **Set multiplicity.** Note cardinality like 1 to many where it matters.
4. **Export.** Download SVG, PNG, or PDF.

## FAQ

### What relationships can I show?

Inheritance, association, aggregation, composition, and dependency, with multiplicity labels — the standard UML class relationships, written as simple Mermaid connectors.

### Can I show fields and methods with visibility?

Yes. List members with +/- for public/private and include types and return values, so the diagram documents the actual interface.

### Is this good for documenting a codebase?

Very — because it's text, the diagram sits next to the code and can be updated in the same commit, avoiding the usual stale-diagram problem.

### Does my class model get uploaded?

No. It renders in your browser, so a diagram of proprietary classes never leaves your machine.


## Related tools

- [ER diagram](https://www.safepaper.app/diagram/er-diagram)
- [Sequence diagram](https://www.safepaper.app/diagram/sequence-diagram)
- [State diagram](https://www.safepaper.app/diagram/state-diagram)
- [Playground](https://www.safepaper.app/diagram/playground)

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Canonical HTML: https://www.safepaper.app/diagram/class-diagram
Markdown: https://www.safepaper.app/diagram/class-diagram.md

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