Storm in.
Plan out.

Most online diagram tools post your text to a server and send a picture back. This page is built so that it cannot. Three ways to confirm that without taking our word for it.

The browser enforces it

The page ships with a Content Security Policy that forbids every outgoing connection. Your browser reads that policy before it runs a single line of our code, and from then on refuses to let the page talk to any server. The page has no way to upload your diagram.

connect-src 'none'
form-action 'none'

Pull the plug

Open the editor, switch off your Wi-Fi, keep drawing. Rendering, converting and exporting all run on your device. Open each format once before going offline so that its renderer is loaded.

Read the code

It is plain, readable JavaScript. Open your browser's developer tools, or “view source”, and look. Every library ships with the page. No fonts, analytics or ads are fetched from anywhere else.

Pick a format and write. The drawing follows every keystroke.

flowchart LR
  A[Idea] --> B{Ready?}
  B -->|yes| C[Ship]

@startuml
User -> App : text
App --> User : SVG
@enduml

digraph G {
  Editor -> Renderer;
  Renderer -> SVG;
}

part def Vehicle {
  part engine : Engine;
}
part def Car :> Vehicle;

<mxCell value="Start"
  style="ellipse;"
  vertex="1">

SysML v2 covers a practical subset of the textual notation: structure, action flows, state machines, use cases and requirements. The draw.io preview is simplified: shapes, colours, labels and arrows are drawn, images and exotic shape libraries are approximated.

Mermaid, PlantUML, Graphviz, draw.io and SysML are names of their respective owners. Stormtoplan is an independent project and is not affiliated with, endorsed by or sponsored by them.

Change the format in the editor and the diagram is converted on the spot. Formats never match exactly, so nothing is dropped quietly: every line that could not be carried over is listed with its line number and the reason. Switch straight back and your original text returns unchanged.

@startuml
skinparam monochrome true
actor User
User -> App : open file
== Phase 2 ==
App --> User : preview
@enduml

sequenceDiagram
  actor User
  participant App
  User->>App: open file
  Note over User,App: Phase 2
  App-->>User: preview

“One way” means the result is a graph or a SysML model that you can keep editing, but it cannot be turned back into the original diagram type; switching straight back in the editor restores your original instead. Graphviz and draw.io convert into each other in both directions, and SysML converts to Graphviz and draw.io as a graph. Towards Mermaid and PlantUML a SysML text becomes a class, state, activity or use case diagram, depending on which kind of element it holds most of; the rest is named in the loss list. Flowcharts with decisions or start and end points become SysML action flows. When no direct route exists, the conversion runs in steps and the loss list covers every step. Types without a counterpart (for example Mermaid timelines or pie charts) are refused with an explanation.

Guides: PlantUML to Mermaid · Mermaid to PlantUML · PlantUML to draw.io · Mermaid to draw.io · PlantUML viewer · Mermaid viewer · draw.io viewer

• PreviewRedrawn while you type.
• TemplatesReady-made starting points for every format.
• DrawingIn draw.io: place shapes, type into them, draw arrows, fix where they attach, bend them, decide what lies in front.
• RequirementsTables of numbered requirements next to the diagram they belong to, and one Markdown spec for a coding AI.
• NavigationZoom and pan the preview, drag the divider, keyboard shortcuts for all of it.
• TabsSeveral diagrams side by side, each with its own format. Rename, reorder and close them.
• Find and suggestionsSearch the labels in the diagram; keywords and snippets in the editor with Ctrl+Space.
• PresentationFullscreen with only the diagram, for showing it to others.
• ExportSVG, PNG, standalone HTML, PDF.
• StorageYour work stays in your browser, tabs included. Open and save .mmd, .puml, .dot, .sysml and .drawio files, or open a Markdown file and edit its diagram blocks in place.
• BackupOne file with all your tabs, and in Chrome and Edge an optional auto-save into a file on your device.
• AccountNone. No sign-up, no paywall, no tracking cookies.
• PriceFree to use.

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