How does Mermaid create diagrams from text and code?

Updated

Mermaid turns a short block of text into a rendered diagram. You describe the diagram with a Markdown-like syntax, and a Mermaid renderer converts that text into a visual chart, so you never position boxes or draw arrows by hand. It fits situations where the diagram should live next to the content it explains — documentation, a README, a wiki page, or a web app — and where you want to edit and version the diagram like any other text file. Note that the project now points to a new home: the site states "Mermaid.js now lives at mermaid.ai."

The core mechanism: text in, diagram out

The workflow has three parts:

  1. You write a diagram definition. A declaration line names the diagram type (for example flowchart, sequenceDiagram, or gantt), followed by lines describing nodes and the relationships between them.
  2. A renderer parses that text. It reads the structure you declared and computes the layout — where shapes go, how lines route, how labels attach.
  3. The result is drawn as a diagram. The output is a visualization, not an image you edited by hand. Change the text, and the picture changes on the next render.

Because the source is plain text, the diagram definition can sit in the same file or repository as the code and documentation it describes, and it moves through the same review and version-control process.

A concrete example

A flowchart definition looks roughly like this:

flowchart LR
    A[Write text] --> B[Renderer parses]
    B --> C[Diagram is drawn]

The first line sets the direction and diagram type. Each following line declares a node (A[Write text]) or a connection (-->). The renderer reads those relationships and produces a left-to-right flow. To add a step, you add a line; to reorder the flow, you change the arrows. No dragging, no alignment.

The same pattern applies across diagram types — a sequence diagram lists participants and messages, a Gantt chart lists tasks and durations. The syntax differs, but the principle is identical: structure in text, layout computed for you.

What this approach is good for

  • Diagrams that change with the content. When the surrounding documentation is edited, the diagram definition is edited in the same place.
  • Reviewable changes. A diagram update appears as a text diff, so a reviewer can see exactly what changed.
  • Embedding. The site lists integrations as a feature, which is the point of a text format: it can be rendered inside tools you already use rather than exported as a static file.
  • Low-friction editing. The site's own framing is "Easy to use" and "Create diagrams and visualizations using text and code" — the value is that describing a diagram takes less effort than drawing one.

Where it is less suitable

  • Highly custom visual design. If you need precise control over styling, spacing, or a bespoke look, a computed layout works against you; a drawing tool gives you direct control.
  • One-off, never-reused diagrams. If a diagram will not be edited or embedded, the text approach adds little over drawing it once.
  • Diagrams whose meaning is spatial rather than relational. When the exact position of elements carries meaning, automatic layout can obscure it.

How to decide

Choose Mermaid when the diagram is documentation — something that should be edited, reviewed, and kept in sync with text. Choose a manual drawing tool when the diagram is a design artifact where appearance and exact placement are the point. The deciding question is whether you want to maintain the diagram as text or as a picture.

The project's page lists a creator, a set of developers, and a contributors section with an invitation to "Join the community and get involved," so the tool is community-developed and open to contribution. For current syntax details and supported diagram types, the authoritative reference is the project's own documentation.