Flowchart vs Sequence Diagram: When to Use Which

Flowcharts and sequence diagrams are the two most common diagram types in software documentation. They look different, answer different questions, and suit different scenarios — but it's not always obvious which one to reach for. This guide breaks down both, with Mermaid.js examples you can copy and adapt.

What Is a Flowchart?

A flowchart models a process. It shows the steps, decisions, and paths through a workflow. The focus is on what happens and in what order, without caring about who or what performs each step.

flowchart TD
    A[Receive order] --> B{In stock?}
    B -->|Yes| C[Pack items]
    C --> D[Ship order]
    D --> E[Send confirmation email]
    B -->|No| F[Notify customer]
    F --> G[Offer alternatives]

Flowcharts answer: "What is the process?" They're ideal for decision trees, onboarding flows, error handling logic, and any scenario where branching paths matter.

What Is a Sequence Diagram?

A sequence diagram models interactions between participants over time. It shows who talks to whom, in what order, and what data is exchanged. The vertical axis represents time flowing downward.

sequenceDiagram
    participant Customer
    participant API
    participant PaymentService
    participant DB

    Customer->>API: POST /checkout
    API->>PaymentService: Charge card
    PaymentService-->>API: Payment confirmed
    API->>DB: Create order record
    DB-->>API: Order ID
    API-->>Customer: 200 OK (order confirmation)

Sequence diagrams answer: "Who communicates with whom, and when?" They're ideal for API call chains, microservice interactions, authentication flows, and debugging distributed systems.

When to Use Which

Side-by-Side: Login Flow

Here's the same feature — user login — documented both ways.

As a Flowchart

Shows the logic and decision points:

flowchart TD
    A[User enters credentials] --> B[Send to server]
    B --> C{Valid?}
    C -->|Yes| D[Create session]
    D --> E[Redirect to dashboard]
    C -->|No| F{Attempts > 3?}
    F -->|Yes| G[Lock account]
    F -->|No| H[Show error message]
    H --> A

As a Sequence Diagram

Shows who talks to whom:

sequenceDiagram
    participant Browser
    participant API
    participant Auth
    participant DB

    Browser->>API: POST /login (email, password)
    API->>Auth: Validate credentials
    Auth->>DB: Lookup user
    DB-->>Auth: User record
    Auth-->>API: Token
    API-->>Browser: 200 + Set-Cookie

Neither is "better" — they document different aspects. In practice, complex features often benefit from both: a flowchart for product/UX review and a sequence diagram for engineering documentation.

Combining Both in Documentation

A common pattern in technical specs:

1. Flowchart first — outline the high-level process so everyone agrees on the logic
2. Sequence diagram second — detail the implementation: which services, what endpoints, what data

Both diagram types work natively in GitHub Markdown — just wrap them in a ```mermaid code fence. For more on embedding diagrams in READMEs, see How to Add Mermaid Diagrams to GitHub READMEs.

Mermaid Syntax Differences

Try It Yourself

Have a diagram screenshot you need as editable code? Upload it and get Mermaid syntax in seconds. Try ImageToMermaid free.