UML sequence diagram maker
Add the participants, click New message and then the sender and the receiver. Drag to change the order.
How to useWho talks to whom, and in what order
Available in: 
UML sequence diagram maker: what it is for
Type the messages exchanged between the participants, one per line, with solid arrows for calls and dashed arrows for replies. Use alt and else for alternative paths, loop for repetitions and notes to explain a step.
The diagram appears next to the text as you type and can be downloaded as PNG or SVG. The code follows Mermaid syntax and also works on GitHub and GitLab.
How to use
- Declare the actors and participants.
- Type the messages in the order they happen.
- Download the diagram as PNG or SVG.
When to use it
- Documenting an API flow.
- Explaining a checkout or login process.
- Software engineering coursework.
Frequently asked questions
How do I write a message?
One line with the sender, the arrow, the receiver and the text after the colon: Website->>API: Sends the order. The ->> arrow is a call and -->> (dashed) is the reply.
How do I show a condition or a loop?
Use alt Condition ... else Other condition ... end for alternative paths, opt for something optional and loop Text ... end for repetitions. Notes go in with Note right of Participant: text.
What is a sequence diagram for?
To show the order of messages between people and systems in a use case: a purchase, a login, an API integration. It helps agree on the flow before coding and to document how the system works.
Is it really free?
Yes, 100% free. No sign-up, no subscription and no usage limits. All Tools123 tools are free and run right in your browser.