Activity diagram maker
Draw a UML activity diagram as text. Show actions, decisions, parallel fork/join and swimlanes with a Mermaid flowchart — then export. The whole thing renders on your device.
One step per line. Indent two spaces to branch beneath the line above.
A UML activity diagram, written as text
An activity diagram is the UML cousin of a flowchart: it shows the flow of work — actions, decisions, and crucially the points where work splits to run in parallel and rejoins. Mermaid does not ship a dedicated activity-diagram type, so the honest, fully renderable approach is a flowchart that follows UML activity conventions: rounded terminators for start and end, rectangles for actions, diamonds for decisions with guarded branches, and a labelled bar node for fork and join.
Because it is a normal Mermaid flowchart underneath, every example on this page renders live and exports cleanly — no plugin, no special mode.
Fork, join and swimlanes
To show concurrency, route several arrows out of a single “fork” node and back into a single “join” node — the diagram then reads as “do these in parallel, then continue once all are done”, which is exactly the UML fork/join semantics. The fork and join bars here are ordinary nodes labelled to look like the UML synchronisation bar.
To assign steps to roles, wrap each role’s actions in a subgraph. These partitions approximate UML swimlanes, making it clear who performs which action — the customer, support, or engineering — without leaving plain text.
Frequently asked questions
How do I make a UML activity diagram?
Use a Mermaid flowchart that follows UML activity conventions: rounded "([Start])" and "([End])" terminators, rectangles for actions, diamonds for decisions, and a labelled bar node for fork/join. It renders live and exports to SVG or PNG.
How do I show parallel actions (fork and join)?
Route several arrows out of one "fork" node and merge them back into one "join" node. That reads as “run these in parallel, then continue once all finish”, which matches UML fork/join semantics.
Can I add swimlanes or partitions?
Yes — wrap each role’s actions in "subgraph Role … end". Each subgraph becomes a labelled lane, approximating UML swimlanes so it is clear who performs each action.
Why use a flowchart instead of a dedicated activity diagram type?
Mermaid has no separate activity-diagram syntax, so a flowchart with UML conventions is the approach that actually renders. You still get every activity-diagram element — actions, decisions, guards, fork/join and lanes — and the source stays portable.
Is my activity diagram uploaded anywhere?
No. It renders entirely in your browser with Mermaid, so your process steps and internal logic never leave your device.