A Sankey diagram is a flow chart where the width of each flow band is proportional to the quantity it carries. The thicker the band, the more of something (money, users, energy) is flowing through that path.
Use the free Sankey diagram maker to turn source, target, and value columns into a proportional flow diagram.
The core idea
Sankey diagrams have one job: show how much moves from one place to another. Where a flowchart shows the steps in a process, a Sankey shows how volume distributes across those steps.
The key insight is visual: wide bands are immediately recognizable as high-volume paths. You don't need to read numbers — the geometry does the work.
Anatomy of a Sankey diagram
- Node — a rectangle or box representing a stage, category, or state. Example: "Website Visitors", "Sign-ups", "Purchases".
- Link / flow — a curved band connecting two nodes. Its width encodes the quantity flowing from source node to target node.
- Source — the node where a flow originates.
- Target — the node where a flow arrives.
- Value — the numerical quantity the flow carries (visitors, dollars, megawatts, kilograms).
A real example: website funnel
Imagine a website with 10,000 monthly visitors:
- 10,000 visit the homepage
- 4,000 click to a product page
- 1,200 add something to cart
- 400 complete a purchase
In a Sankey diagram, the band from "Homepage" to "Product page" is thick (4,000), the band from "Product page" to "Cart" is thinner (1,200), and the band from "Cart" to "Purchase" is thin (400). The missing volume — the users who left at each step — can be shown as flows to an "Exit" node.
At a glance, you see where you lose the most potential customers.
Where Sankey diagrams come from
Sankey diagrams are named after Irish captain Matthew Henry Phineas Riall Sankey, who used them in 1898 to visualize the energy efficiency of a steam engine. The original diagram showed how much coal energy was converted to useful work versus lost as heat. The format was so effective at communicating waste that it spread across engineering, economics, and eventually data science.
Common use cases
- Website funnels — traffic → sign-ups → purchases → retention. Shows drop-off at each stage.
- Energy flows — electricity generation from fuel sources to end uses and losses.
- Budget allocation — revenue flowing to salaries, operations, profit, taxes.
- Material flows — raw materials through manufacturing to products and waste.
- User journey maps — how users move between product features or states.
What Sankey diagrams reveal
The shape of a Sankey makes certain patterns immediately visible:
- A wide band suddenly narrowing → significant volume loss or drop-off at that step
- One path much wider than others → dominant flow route; most volume goes here
- Many thin bands from one node → high fragmentation; volume disperses widely
- A thin band surviving all stages → rare but consistent path through the system
Sankey diagram vs pie chart
Both show parts of a whole, but Sankey diagrams show how those parts flow and transform:
| Sankey diagram | Pie chart | |
|---|---|---|
| Shows flow direction | Yes | No |
| Shows multiple stages | Yes | No (one snapshot) |
| Encodes quantity in width | Yes | Yes (angle/area) |
| Good for funnels | Yes | No |
| Good for simple composition | Overkill | Yes |
Use a pie chart when you have one composition question. Use a Sankey when you need to show what happens between stages.
FAQ
Can flows loop back in a Sankey diagram?
Standard Sankey diagrams are acyclic — flows only move in one direction (left to right, or top to bottom). Loops would make the diagram impossible to read. If your data has cycles, consider a chord diagram or a network graph instead.
What's the difference between a Sankey diagram and a flow chart?
A flowchart shows the logic of a process — decisions, branches, steps. A Sankey diagram shows the volume flowing through a process. Flowcharts are for process documentation; Sankey diagrams are for quantitative flow analysis.
How many nodes can a Sankey diagram have?
Readable Sankey diagrams typically have 5–15 nodes. Beyond 15–20 nodes, the overlapping flows become visual spaghetti. Group minor categories into an "Other" node to keep the diagram clean.
How this guide is reviewed
This guide is maintained by the ChartsMakers Editorial Team and reviewed for statistical accuracy, product behavior, and clarity. When a guide relies on software behavior or statistical definitions, we check it against official documentation or established references. See our editorial policy.