Sankey diagram vs flow chart: they answer different questions

The two get confused constantly because both draw boxes connected by lines going left to right. The difference is what the lines mean.

A flow chart is a map of possibility

In a flow chart, an arrow means this can lead to that. Every arrow is the same weight because possibility has no size. Decision diamonds, loops and terminators all exist to describe logic, not volume. That makes flow charts right for processes, onboarding steps and system design.

A Sankey is a map of measurement

In a Sankey, an arrow means this much of that became this. Arrows cannot be equal weight — that is the whole point. It follows that a Sankey cannot represent a loop or a conditional, only quantities moving forward through stages.

The test that settles it

Ask whether you could put a number on every connection. If yes, and those numbers add up at each box, you want a Sankey. If the connections are steps rather than amounts, you want a flow chart and a Sankey will look absurd.

• Checkout process design → flow chart.
• How many users completed each checkout step → Sankey.
• Approval rules for expenses → flow chart.
• Where the expense budget actually went → Sankey.

The two formats often pair well: draw the flow chart to agree on the process, then draw the Sankey to show what the process actually produced last quarter.